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Pinterest, the social media site where people build collections, said this weekit will start taking paid ads for the first time. They're called "Promoted Pins," and look a lot like the other content on the site.

With millions of users, will this be Pinterest's golden ticket? For a company valued at nearly $4 billion, but still not making money, selling ads is a smart move, says Noah Abelson, CEO of Shareroot, which works with brands to promote themselves on Pinterest.

"You express yourself through what you're interested in," Abelson says.

And because Pinterest is about interests, he says, "It's very, very easy for a brand to be a part of that conversation."

General Mills, Target, and Banana Republic are among the companies buying promoted pins. The price could be more than $1 million, depending on how many people the ads reach. Pinterest has tens of millions of users.

"So if you're a cosmetics company or a fashion company, it could be very, very effective to advertise on Pinterest," says Karsten Weide an analyst with IDC. On the other hand, he says Pinterest has a limited audience: mostly younger women, with kids.