NEW YORK – Rapper Snoop Dogg gave props on Twitter to an ad for the Toyota Sienna minivan. Actress Tori Spelling linked to a website for rental cars. And reality TV star Khloe Kardashian soliloquized about the brand of jeans that accentuates the famous Kardashian derriere.

“Want to know how Old Navy makes your butt look scary good? Ask a Kardashian,” the reality TV star wrote, or tweeted, on the social media website. Of course, she capped off the reflection with a smiley face.

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Hip-hop artist Snoop Dogg is one of the celebrities who have been paid to promote a product on Twitter. He tweets about the Toyota Sienna minivan. Other stars include actress Tori Spelling tweeting about rental cars and reality TV star Khloe Kardashian who pushes a brand of jeans. The Associated Press

These celebs aren’t just writing about family cars and fashion choices for the heck of it. Stars can get paid big bucks – sometimes $10,000 or more per post – to pontificate about clothes, cars and movies in the 140 characters or less allowed per tweet. That adds up to about $71 per character.

Twitter, which in its five-year existence has reshaped how people shop, vote and start revolutions, is now changing the business of celebrity endorsements. Just as Match.com and eHarmony pair up singles for dates, a growing number of startup firms are hooking up companies with stars who get paid to praise products to their thousands – sometimes millions – of Twitter followers.

The list of celebs and the things they hawk is long and getting longer all the time. The endorsements range from subtle to blatant; the celeb pairings from sensible to downright odd.

Singer Ray J urged his 600,000-plus Twitter followers to see the horror movie “Saw 3D.” Football star Terrell Owens gave a shout-out in front of his more than 1 million followers to a hotel chain giving away sports tickets: “Comfort Inn is hooking up 3 days of it!” Lamar Odom, the L.A. Lakers forward, tweeted to his nearly 2 million followers about hip-hop artist and entrepreneur Jay-Z’s book “Decoded”: “My man Jay-Z … only rapper to rewrite history without a pen. Until now.”

Of course, anything on Twitter is short-lived and reaches only a small, self-selecting audience: Research firm eMarketer estimates that only 11 percent of U.S. adult Internet users are on the micro-blogging site. And even though some celebs have faithful groups of followers, it can be hard to measure whether their tweets lead people to spend.

Still, celeb tweets can be a way to grab an audience at a time when many people skip TV commercials using their digital video recorders. And paying a celeb to tweet is much cheaper than a traditional advertising campaign. Want a tweet from Khloe Kardashian? That will cost about $8,000, according to prices listed by social media marketer Izea. Looking for a cheaper option? Ray J is about $2,300.

Companies like Izea, Ad.ly and twtMob usually pair products with celebs through a combination of software algorithms and Hollywood instinct. The companies say they use many metrics to gauge the effectiveness of a paid tweet, such as the number of times it gets reposted by others.

When Ad.ly got Charlie Sheen to tweet for Internships.com in March, the actor was in the midst of getting fired from his sitcom “Two and a Half Men” over accusations of hard partying and drug use. Within an hour of Sheen’s first post, Internships.com got more than 95,000 clicks.

Like any endorsement, celeb tweets come with the risk that a star’s behavior will not coincide with the company’s image. And of course, there’s a science to picking the right one: Will consumers buy that their favorite rapper drives a minivan?

Twitter generally allows the paid tweets, as long as they’re posted manually and not automated by a computer program. The Federal Trade Commission suggests endorsers end their tweets with the hash symbol, called a hash tag, and the letters “ad” or “spon,” short for “sponsored by,” to clarify that they’re ads.

“The more transparent you are with your audience on Twitter, the more powerful that connection is,” said Rachael Horwitz, a company spokeswoman.

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