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LOS ANGELES (AP) — It isn’t just “tiger blood” fueling Charlie Sheen’s tweeting: There’s a new world of social media advertising hidden behind his micro-blogging, too.

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KNX 1070’s Ed Mertz Reports

Shortly before Warner Bros. Television announced Monday that it had fired Sheen from the CBS hit sitcom “Two and a Half Men,” the actor tweeted that he wants to hire an intern to assist him. But the tweet also was a paid-for endorsement from the year-old website Internships.com, which claims to be the world’s largest internship marketplace.

“I’m looking to hire a winning INTERN with TigerBlood,” read the message, with a link to the site.

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The job posting on Internships.com describes the position as a paid, eight-week job for the summer to “work closely with Charlie Sheen in leveraging his social network.”

The partnership was arranged by startup ad.ly, which connects celebrities with advertisers for social media ads. Sheen officially signed on with ad.ly after the Beverly Hills, Calif.-based company consulted with him last week to familiarize him with Twitter.

Ad.ly earlier helped Sheen join Twitter and get his account immediately verified so that users could separate the real Sheen from the many copycat accounts. Sheen took little more than a day to reach 1 million followers, a record. He had more than 2 million followers as of early Tuesday.

All the attention has brought a huge amount of exposure to the business of social media advertising. Though companies have been working advertisements into Twitter and Facebook for more than two years, it’s a sometimes unnoticed practice.

“A lot of people know about the business now,” says Ad.ly CEO Arnie Gullov-Singh. “It’s a validation of the business that we’re building and the overall industry changes that we’re a part of.”

Micro-endorsements can net a celebrity anywhere from $1,000 to the low five figures per tweet, with ad.ly’s top celebrities earning about $10,000 per tweet. Ad.ly and Internships.com declined to discuss the financial arrangements of the deal with Sheen. Pricing is frequently structured on the number of clicks an advertiser gets via the ad, with $1 to $2 per click.

Ad.ly’s roster of celebrities with whom it has worked includes Mariah Carey, Kim Kardashian, Chris Brown, 50 Cent, Paris Hilton and soccer star Cristiano Ronaldo. In the last year, it has placed 24,000 endorsements for 150 brands and raised more than $6 million in venture capital.

Last year, ad.ly was behind a tweet sent by Snoop Dogg, sponsoring Toyota: “These homies know the deal. Wonder if this swagger wagon can fit 22’s? SPINNIN!”

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A staff of 22 includes several workers who specialize in writing such tweets in the voice of the respective celebrity.

Gullov-Singh says he sees celebrities as “the driving force of social media.”

“Brands want to get their arms around social media, and celebrities are social media,” Gullov-Singh says. “So that’s where we set off to connect the dots and create value.”

Asked whether Internships.com had any reservations about partnering with Sheen, CEO Robin Richards replied, “Charlie Sheen is an A-list actor for seven years in a row.”

“We thought we could really highlight and help students and companies realize that this resource was available for them,” says Richards, who also is an ad.ly board member.

There are several other start-ups with a similar focus, including Izea, Assetize and Magpie. But it’s also not only a specialized business, with many individuals and companies making arrangements without third parties.

The Orlando, Fla.-based Izea, founded in 2006, put together a 12-day campaign for Microsoft with Sean “Diddy” Combs sending a Microsoft-tagged message once a day for a week on Facebook and Twitter. It included giveaways, charity donations and an Xbox contest.

“It’s got to be relevant to someone’s fans or someone’s followers,” says Dan Rua, chairman of Izea. “The driving effort is really: Make sure the conversation is relevant to the people involved. If you’re giving quality, relevant content that gets people excited, then you’re going to do all right.”

Advertising in social media, though, has its risks. One of social media’s most popular characteristics is its seemingly unfettered access to celebrities. Though ads are required to be disclosed, any relationship becomes less personal once someone is hawking Toyotas. But at the same time, entertainers supply a wealth of content to their fans on social media for free.

“There’s always a group of people that think everything should be free. That’s just naJive,” Gullov-Singh says. “When we started ad.ly, our friends at Google looked at us scornfully and said, ‘How can you be polluting the stream?’ We said, ‘Dude, look in the mirror. Look at your company. Your entire business is based on putting ads on everything.”‘

Celebrities are creating content and have a right to “monetize it,” Gullov-Singh adds.

“Otherwise,” he says, “why should they do it?”

» PHOTOS: Is Charlie Sheen Really ‘Winning?’
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