Pinterest has over 11m monthly visitors in the US

"It’s beautiful, it’s addictive, and now Pinterest is having its glorious hockey stick moment. TechCrunch has attained exclusive data from comScore showing Pinterest just hit 11.7 million unique monthly U.S. visitors, crossing the 10 million mark faster than any other standalone site in history.
In fact, users are spending so much time sharing their favorite images that now only Facebook and Tumblr have more social media time on site than Pinterest. Who’s propelling its rise? 18-34 year old upper income women from the American heartland. Maybe we should call it blow-dryer growth.
According to comScore, the average Pinterest user spend 98 minutes per month on the site, compared to 2.5 hours on Tumblr, and 7 hours on Facebook. If you don’t count sites like Google+ or new Yahoo channels that have built in user bases, comScore’s data shows Pinterest would be the fastest site of any kind to hit 10 million monthly uniques in the U.S.
[Update: comScore reported to us that Pinterest was the fastest independent site to hit 10 million monthly uniques in the U.S., and that according to its data, Formspring never reached 10 million monthly active users in the U.S. and instead peaked around 8+ million."

Source:  TechCrunch, 7th February 2012