If you tweet for free, you must not be a celebrity. Depending on how many followers they have, celebs can earn thousands of dollars just for sending out a message on Twitter.

The biggest player in the pay-to-tweet market is Ad.ly, a social-media advertorial firm that matches brands with celebs, resulting in highly-personalized tweets that shill a variety of companies.

So who's raking it in?

Kim Kardashian, who never met an endorsement opportunity she didn't love, is reportedly bringing in $10,000 every time she mentions ShoeDazzle.com. Rapper Snoop Dogg, who has 6.4 million followers, gets $8,000 or more per tweet. Paula Abdul's 2.2 million followers means she earns $5,000 per tweeted ad. Even Whitney Port, star of MTV's 'The Hills,' is paid $2,500 for tweeting an ad to her 800,000 followers.

But those are all chump-change compared to the $50,000 Charlie Sheen got when he tweeted out a call for a "Tiger Blood" intern for Internships.com last year. It generated 450,000 clicks in its first 48 hours, made #tigerbloodintern a worldwide trending topic, and brought in 1 million site visits.

Assuming he maxed out the 140-character limit, that's still almost $338 per letter (or space). What's more, he probably didn't even write the tweet. Ghostwriters or ad people typically do the typing for celebrities.

Regardless, says Ad.ly CEO Arnie Gullov-Singh, “Consumers react positively to organic authentic messaging.” Which explains why Kanye West pretty much Tweets whatever pops into his head.

[via NYMag]