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Magpie, the AdSense-like service that sells product placement in Twitter users' tweets, put itself up for sale today.

The service brags that its advertising network now encompasses 20 million Twitter users and about 4,700 advertisers, including Sony Playstation, Microsoft Xbox, Burger King, Snickers, Honda, Audi, Mazda, Heineken, Bacardi and Hersheys. Magpie is a raging success, the founders say, but they have decided to move on.

According to Download Squad, four days ago Cheryl Smith's picture appeared next to a Facebook ad targeted towards her husband. The copy was not only inaccurate, but a little disturbing, "Hey Peter, Hot singles are waiting for you." As far as we know the Smiths are not into kinky role play and Facebook has not built an amazing new relationship compatibility algorithm. In actuality, Cheryl is the unfortunate victim of the fact that Facebook Facebook's 3rd party application developers have used our mugs to advertise to our friends.

What are you doing? No what are you doing Apple, Skype, Flip, StubHub and Box.net?? These popular companies just couldn't resist paying off Twitter users to put advertisements into their Twitter streams using the new pay-per-tweet service Magpie. It's enough to make you question the true motives of any outspoken fan and the end result is pretty laughable when you take "a bird's eye view."

We were disappointed when a browser script showed us a Magpie redirect behind a shortened link in a Skype testimonial today. Then we used a search on the service BackTweets to find out who else is buying fake Tweets on the service. It's so revolting and pitiful that it's kind of sad.

While Twitter has been less than forthcoming on how they plan to monetize their service, there is no shortage of ideas from third parties on ways to get paid for spending time with Twitter. From pay-to-tweet to selling off the real estate on your Twitter background, there are any number of ways you could be making money off the service.

Now, there's another service that - much like RSS-based advertising - offers to pay you for advertisements that run in the midst of your tweet stream. Meet Magpie, an ad network for Twitter.