Slacker is in growth mode. With those 30 million members come almost 400,000 paid premium subscriptions. The company is able to convert about 10% of new mobile free subscribers to paid members, a robust number. Yet, it is important for Slacker to optimize its streaming product as 75% to 85% of listening on the service comes from the free streaming product and not the paid on-demand option. That is where YuMe comes in. With Spotify on the rise and Pandora the dominant service in the sector, Slacker has to make as many moves as it can to gain an edge.

It also does free mobile streaming, which makes it a competitor with Rhapsody, Pandora and Rdio. Music on the Web has been an ultra-competitive market since Napster (which still exists through Best Buy) broke the traditional music industry model in the early 2000s. Yet, Slacker thinks that it can take what made the traditional model so successful and apply it to the new era of music online.

Traditional Model Applied To Digital Era

Slacker's radio channels are curated by a 80 or so "experts" that the company thinks of as traditional disc jockeys. The idea is to have a mix of people's favorite music coupled with new or obscure artists for users to discover. Once a user hears something they might want to buy, the Slacker premium model is there to fulfill that purchase. In this way it is no different from the music industry of the 1950s to 1990s when people listened to the radio and then ran to the record store to buy the newest album.

Yet, free music on the radio has never really existed without ads. Even Pandora, which started sans-advertising, now has ads (or a premium model without) and it has carried the company to a moderately successful IPO. YuMe can help Slacker with this and the two companies are uniquely suited for each other. Both are mobile first and both have services optimized for iOS, Android and Blackberry (Slacker also does Windows Phone 7 and Symbian).

Slacker is only going to get bigger. It has made a deal to be the primary music streaming service for AOL, a move that the company thinks will double its subscription base. The conversion rate to premium memberships will undoubtedly be lower but the more ears Slacker can get, the innovation it can bring to the space and content licenses it can acquire. Will it be enough to separate it from the likes of Mog, Rdio and Rhapsody (and now Spotify) and approach Pandora levels? That remains to be seen.

To ditch the advertisements and skip songs you don't like, a $3.99/month subscription (Slacker Radio Plus) was made available. And today, Slacker has launched another option: a $9.99/month Premium Radio subscription for playing the songs, albums or artists you want to hear on demand. This is similar to a number of other services out there today, including two of our favorites, MOG and Rdio.

Which one is right for you?

MOG and Rdio are only two of the cloud-based streaming music subscription services available today, so this isn't meant to be a comprehensive review. Others, like Napster, Grooveshark, Spotify, Rhapsody, Microsoft's Zune and others are also available, with similar pricing.

But MOG, Rdio and Slacker are, let's be honest, some of the newer, and cooler* services** these days (for U.S. users***), each offering unique features, from social sharing of playlists to free cloud storage for your own tracks and more.

*Cool, as you know is an entirely subjective word. Did we mention this was a blog?

**We think Grooveshark is pretty cool too, but is still facing questions about its legality. The company maintains there's nothing illegal about its service, but in truth, it's operating in a gray area of the law and has since been pulled from iTunes because of that. Spotify may be the coolest of the bunch, but is not available commercially in the U.S. market. Napster and Rhapsody are nice too, but are old brands doing new things, as opposed to new startups doing new things. That doesn't make their services unworthy of consideration, but they are not being reviewed in this article.

***Yes, this review is U.S.-centric, sorry overseas readers!

What is Slacker Premium Offering?

With Slacker, the new Premium service offers over 8 million tracks from the four major labels and thousands of indies. You can search, play and replay specific songs or entire albums. And you can create playlists both online and on your mobile. Currently, Slacker has apps for iPhone, iPod Touch, iPad, Android and BlackBerry phones. Offline playback is supported on these devices, too.

Users can also access the 150+ pre-programmed Slacker Radio stations based on musical genre, and can view and play the top 50 most popular songs for that genre's station.

While listening to music, users can favorite tracks for easy retrieval later. And like the Plus offering, this service is available ad-free, and with unlimited song-skipping. It even offers the ability to view song lyrics.

Slacker notes on the subscriptions page of its website that with 8 million tracks, Slacker has 6 times more songs available than its top competitor. That's a fair comparison, given that Pandora's catalog only includes 800,000 unique songs. While not all of Slacker's 8 million tracks are available on demand - that's up to the labels' discretion - all those that are available on demand on competing services would be available on Slacker, too.

Although Slacker does trump Pandora's catalog hands-down, the service's real competitors are not other "radio" applications, but on-demand subscription services like Rdio and MOG.

Here, the competition is a bit tougher.

How Music Discovery & Management Compares

With MOG, for example, you have access to features similar to Slacker - favorite tracks on demand, top charts, playlists and more. But while Slacker breaks down the charts into "top 50's" by genre, MOG offers "Top 50" charts for all music being played on its service, available by song, artist or album. (This is on its mobile app, to be clear).

The serendipitous discovery on MOG comes instead from a curated "Editor's Picks" playlist and "Featured Playlists" from other MOG users. There are also featured "Radio Picks" if you prefer the radio format over the on-demand format.

From the Web, you can favorite other MOG users' playlists or build your own and these then become available on your mobile. But in the mobile app itself (iPhone and Android only), you can only favorite a track or artist. You can't add a given track to a playlist on the fly, as you can on Slacker.

Meanwhile, on Rdio, the emphasis is more on social discovery of music through friends rather than through "editors" or strangers with similar interests. In a "recent activity" section on the mobile app's dashboard, you can see your friends' activity on Rdio, including music they've added or who they're following. This is a decidedly hit-or-miss experience for discovery of new tunes, however, unless all your friends like almost exactly the same music you do (which is never the case, we've found).

Rdio also features a "heavy rotation" section so you can see what's popular on the service overall, but this is more limited a snapshot than what MOG and Slacker offer. There are not multiple charts, by genre or otherwise.

On Rdio's iPhone app, a recommended music section has been added, offering a somewhat iTunes-esque "genius-like" list of suggestions based on the songs you've already played. Unfortunately, this same feature has not yet made it to all other mobile platforms yet. (Rdio is available on iPhone, Android, Windows Phone 7 and BlackBerry.)

However, Rdio users can save tracks to their "collection" (an online music collection built using the songs already in your iTunes media library), and they can add tunes to playlists from the mobile app. That means MOG is the only one of the three without playlist management from mobile. For what it's worth, Rdio's app is also one of the better designed streaming music mobile applications available today.

Catalogs! Who Has Your Tunes?

Slacker touts a catalog of over 8 million tracks, and Rdio says it has 8.5 million tracks. MOG, however, has recently bumped up its catalog to offer 11 million songs, making it the largest of the three we're examining today. (For comparison's sake, iTunes now offers 13 million songs, Napster has 10 million songs, Rhapsody, 10 million, Grooveshark, 6 million, Spotify, 13 million.)

All three streaming services say they offer songs from the major labels, however, so these shortcomings will only affect users with more indie or esoteric tastes. Much of MOG's indie selection comes from major indies (Beggars Group, Dischord Records, Matador, Thrill Jockey, Merge Records, Domino, Warp Records) and major indie aggregators (ADA, Redeye, RED, IODAlliance, INgrooves, Virtual Label Group, Finetunes, IRIS, CDBaby - self-released artists, and Tunecore - self-released artists).*

*Yes, those names only mean something to you if you're really into indie music.

Though MOG's app isn't the best in terms of its design, or even features (due to its lack of playlist management on mobile), the ability to stream the most songs may be its top selling point.

So what's Slacker's main attraction then? It doesn't have MOG's catalog or Rdio's deep social sharing integrations, but it does offer a lot of "top 50" stations - which is great for those who want to explore music based on popularity, instead of those who already know what they're after.

Depending on your mobile platform of choice, and the features that you value most - social, design, catalog or discovery - one of these three can easily fulfill your needs. If not, there are the others we mentioned above, plus iTunes of course...and torrents, we suppose, if you're naughty.

As we noted last month, in the battle between the large streaming music services, we are getting to the point where most of these services offer very similar features. Instead of features, we will likely see these services use the size of their music catalog as their main way to differentiate themselves from their competitors. According to a spokesperson for Slacker, the company will offer four times as many songs for on-demand listening as its "leading competitor."

2. Deezer: Deezer offers users free and legal streaming music while sharing advertising revenue with artists and rights owners. The site launched with a Sony BMG partnership and signed a Universal Music deal in 2008. Users can share their favorite music by connecting with friends within the social network, or embedding playlists in 3rd party sites.

3. Spotify: Heralded as one of the best music streaming experiences on the market, Spotify is only available in the UK, Sweden, Norway, Spain, France and Finland. TechDigest TV uploaded a fantastic looking preview of Spotify's much anticipated iPhone app.

4. Tunerec: Swedish company Tunerec allows users to create music libraries and playlists from recorded radio play. Because libraries are taken from recorded music, it takes a while to populate playlists; however, according to RWW's initial review by Frederic Lardinois, the service is worth the wait.

5. Last.FM: If you haven't heard of Last.FM, you've probably been living under a rock. The site offers users the ability to create radio stations and stream them complete with AudioScrobbler-powered recommendations.

6. Pandora: To the user, Pandora and Last.FM are similar recommendation-based radio services; however, where AudioScrobbler makes statistical inferences, Pandora's recommendations are determined by the Music Genome Project's 400 distinct musical characteristics.

7. Slacker: Slacker is another popular radio recommendation service. Users input tracks and receive recommendations. Slacker first launched with custom mobile hardware and has since expanded onto other mobile devices.

8. The Hype Machine: This is a fantastic service for those willing to leave music selection to the experts. Like other sites, this one allows listeners to search for music and stream playlists; however, the files on the site are actually streamed from the blogs of top labels, DJs, promoters and music start ups.

9. Blip.fm: Blip.fm is another site where music lovers can access millions of streaming songs. Members receive their own station and the ability to share station programming responsibilities with friends. The site also offers integration with blogs, Twitter, FriendFeed and Last.fm. The act of blipping refers to the act of linking to a song and attaching a 150 character comment to it.

10. MOG: MOG is a music blog network that encompasses more than 300 blog posts per week. The site offers an in-depth look at new artists and includes music recommendations, videos and streaming audio clips. A good place to start with this service is to play audio from it's Recently Popular Posts page.

11. Lala: Lala also offers users a playable web browser interface. The service contains 7 million free online songs and the ability to purchase additional web songs at 10 cents each or downloadable MP3's for 80 cents and up each.

12. Imeem: Imeem is considered "the new social mixtape". The streaming music site allows users to create playlists and share them across the web. RWW recently covered Imeem's iPhone and Android launch.

13. SoundCloud: SoundCloud also allows users to upload tracks and share them via the cloud. Listeners receive shared files via an email-style interface. From there, they can choose to either play the music from the site or download the tracks they've received from friends.

14. 8Tracks: This service lets users upload 8 tracks as a playlist and share the playlist with friends. This service is essentially what Muxtape used to be.

15. Muxtape: Muxtape has transformed from one of the early mixtape-style music sites (users uploaded and shared playlists) to a directory of bands. It remains a great place to discover indie bands.

16. Project Playlist: Project Playlist indexes music from across the web. Again, users create playlists and share links to music files with their friends. Reviewers see this as one of the best music search engines in existence.

17. Skreemr: Skreemr is also a search engine and music indexing site. It claims to offer users access to "6 million mp3 files from over 100,000 web sites".

18. Fizy: Similar to the now defunct Seeqpod, Fizy is an extremely bare bones approach to streaming music with a simple search bar. Like Seeqpod, the site offers speedy music video results and audio results, and unfortunately, legally questionable content. Perhaps the site's recent acquisition will change that.

On the Horizon: Microsoft is set to launch a streaming music site at the end of July. For more info on this project check out our coverage.

The service now also shows display ads on its website, which, to be honest, don't seem to fit into the general design of the site and look like they were just added for the sake of it.

In the Bloomberg interview, Westergreen also acknowledged that Pandora's struggle with the music industry to negotiate royalty rates could still stop the company from becoming profitable, though Westergreen also said that he is optimistic that these negotiations will come to a positive conclusion for Pandora.

Upload with AIR - Play Over the Air

You upload your music with an Adobe AIR app which allows you to upload songs directly from your hard-drive, but also has a convenient option that lets you browse your iTunes library to pick songs you want to transfer to imeem. In our tests those uploads worked just as expected and went relatively fast. The sounds quality was also surprisingly good.

Similar to the iPhone apps from Pandora, Last.fm, and and Slacker Radio, you can also easily create custom radio stations, though the functions here feel a bit limited compared to the options that imeem competitors now offer.

Like its competitors, the app suffers from the fact that it can't play in the background on the iPhone. Sadly, imeem also doesn't remember where it left off after you exit the app.

Pricing

You can store up to 100 songs, 10 videos, and an unlimited amount of photos on imeem for free. For $30 a year, you can upload up to 1000 songs and 100 videos, and $100 a year buys you storage for 20,000 songs and 500 videos.

Alternatives

We are also big fans of Lala, which allows you to listen to all of your songs online without any restrictions. Sadly, though, Lala does not offer a mobile application.

If you want full access to the music on your desktop while on the go, Simplify Music 2.0 also does a great job at streaming your collection over WiFi, Edge, and 3G. One of the nice features of the Simplify Music app (iTunes link) is that it gives users easy access to lyrics and artist biographies. Imeem only provides access to artist biographies.

Verdict

Given the quality of the apps that imeem's competitors have released, this app still has a bit to go before it can fulfill all of its promises. However, if you are looking for a simple way to expand the capacity of your iPhone or Android phone then imeem might be worth a look, especially if you don't keep your music on a home server where you can easily access it with Simplify Media or Simplify Music.

Overall, Slacker feels a lot like satellite radio, which is probably no surprise, given that XM Radio's co-founder Lon Levin is Slacker's Senior Advisor.

With regards to its user interface, every Pandora and Last.fm user will feel right at home. One nice addition to Slacker, though, is the ability to see which songs will play next.

Customization

One advantage of Slacker over its competitors is that it gives you more options to directly customize your stations. You can direct it to play songs from different decades, and decide if you want it to mostly play hits or also more obscure songs.

Competition

Slacker's reviews and biographies can't quite compete with the depth of information offered by the Last.fm app, which can also display tour dates. Slacker also doesn't feature any of the social networking functions that have made Last.fm so popular.

Just like its competitors, Slacker only allows you to skip six songs per station every hour. Unlike Pandora and Last.fm, however, Slacker does play 30 second radio ads after every fifth song. For $3.99 a month, however, you can buy a premium membership that lets you skip songs as often as you want and which removes the advertising from your stream.

Like all the other streaming music apps on the iPhone, Slacker can't play in the background while you surf the web or read your email. This, of course, is a limitation that Apple has put on its developers, even though Apple's own music player on the iPhone does this without causing any problems.

Verdict

Even though Slacker Radio is very similar to the Last.fm and Pandora app, we think it is definitely worth a try. Whether you prefer one over the other will depend on how happy you are with the music recommendations it gives and how important the social features of Last.fm are to you.