According to a new study from Nielsen, Internet users spend more time on social networks and blogs in May 2009 than ever before. The total number of minutes increased 82% year-over-year. Unsurprisingly, Twitter saw the largest gain in total new users among social networks, with a 1,448% increase in visitors from May 2008 to May 2009. Users also started to spend far more time on Twitter in recent months. In May 2008, the average user spent about 6 minutes on Twitter.com, while this number has now grown to more than 17 minutes (Note: as far as we can see these numbers don't seem to take users who use third-party clients into account).
It is important to note, though, that Twitter's growth has slowed down dramatically over the last two months, as both the time per person spent on the site and Twitter's month-over-month growth only increased slightly since April 2009.

Nielsen also released some new data about Facebook and MySpace. Interestingly, MySpace fired a large number of employees today, but Nielsen found that while Facebook is the top global social networking destination with over 144.3 million unique visitors in May 2009 that MySpace still seems to have at least once niche where it is performing exceptionally well: video. Nielsen found that MySpace still ranks as the top social networking site when ranked by video streams (116.1 million streams in May) and the number of unique video viewers on MySpace grew 22.9% month-over-month from April to May 2009.

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If you follow people with similar interests as you and are interested in what they have to say, you can spend hours on twitter compared to if you were following just random people that post about when they take their dog out, when they're at the bar, etc.
To turn occasional visitors into regular users, Twitter has to do a better job integrating new users into its network. Right now, it takes a great deal of effort to find other users with similar interests (unlike the geographic/school/fan networks & FOAF system on Facebook).
As long as Twitter sees itself as merely an information network for news bulletins, visitors will come & go and it'll just be an additional source for news and links like GOogle Reader. But if it stops denying it is a social network and integrates some of the third party tools that allow people to find each other, it can build something that is more than just a flash in the pan.
Soon enough, there will be more reliable, alternative sources of real-time news than Twitter. But social networks aren't easy to recreate elsewhere. Twitter (and its investors) needs to stop focusing so much on Search & more on user retention and satisfaction if it wants to create sustained & long-lasting growth.
Social media is not just a tool anymore, it's a way of living. The hours you spend online really depends on how involved you are, how much you are willing to share, and mostly do you do it for pleasure or is it just another way you can raise awareness for your business.
Monika Lorincz
monika at surchur.com
http://surchur.com/
Blog: http://blog.surchur.com/
Twitter: @surchur
Yes, Twitter needs to focus on it's search for people function. It can be extremely excruciating to find interesting people to follow.
If you are interested in Singapore or are from Singapore this is a very handy list of ten people to follow.
http://sg.theasianparent.com/articles/01_Singaporeans_to_Follow_on_Twitter_012