Story highlights Pinterest is now the third most popular online social network, a report says

Only Facebook and Twitter get more traffic than the "pinning" site

President Barack Obama recently joined Pinterest

The ranking doesn't include mobile traffic

Think back six months. You probably never had heard of a little website called Pinterest.

Now it's the third most-visited social-networking site in the United States, according to a report released Thursday by Experian Marketing Services, a digital marketing firm.

Pinterest, which lets its users "pin" photos and info from the Internet onto virtual boards, ranks behind only Facebook and Twitter in terms of total visitors, according to the analysis, titled " The 2012 Digital Marketer: Benchmark and Trend Report ."

The ranking is based on the total number of U.S. visitors during March and does not include mobile traffic, according to Experian spokeswoman Jennifer Marshall.

Last month, Facebook had more than 7 billion total visitors; Twitter had 182 million; and Pinterest had 104 million total visits from people in the United States, according to data sent to CNN by Experian.

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That ranking puts the newbie site ahead of heavyweights such as LinkedIn, Google+, MySpace and Tumblr.

"The site has really just rocketed," said Matt Tatham, another spokesman from Experian. "It's just been tremendous since (Pinterest) took off around October and then in the last few months. With Pinterest, it's kind of a new take on an old thing. Social networking is great. Pinterest is great. The way people are sharing on Pinterest is new."

One caveat: Since the data doesn't include mobile traffic, sites such as Twitter, which sees much of its traffic from smartphones and tablets, may take a hit in this ranking, Tatham said.

Pinterest's traffic jumped 50% between January and February. The report calls the site "the hottest social media start-up since Facebook and YouTube."

Those stats add momentum to a site that already had become one of the hottest topics of conversation on tech blogs and was known to be one of the fastest-growing networks.

In February, Pinterest was the third-fastest-growing site on the Internet in the United States, with 17.8 million unique visitors that month, compared with 11.7 million in January, according to a report from another Internet tracking company, comScore.

Pinterest launched in March 2010, but it has grown rapidly only in the past six months. Unlike many social-media darlings, tech bloggers in Silicon Valley largely ignored the site until they noticed that it was growing like mad.

"It's a really humbling feeling that all these people are using something that you helped make," Silbermann said in an on-stage Q&A at last month's South by Southwest Interactive conference.

At that conference, he announced that an iPad app and new pinboards were coming soon.

Those boards -- where people pin photos of products they'd like to buy and other interesting bits of info they find while trolling the Internet -- are key to Pinterest's success, he said.

"To me, boards are a very human way of seeing the world," he said at SXSW. "The site is about helping people to discover things they didn't know they wanted -- things that feel like they've been handpicked just for you."

Some other Pinterest-y stats for you to chew on:

The site skews female -- about 60% of users are women -- and to the middle of the country, according to the report from Experian. Pinterest is most popular relative to other social networks in Missouri, Utah, Alabama, Oklahoma and Kansas, the report said.

Are you on Pinterest? Why or why not? Let us know what you think in the comments.