![Pinterest is Now the No. 3 Social Network in the U.S. [STUDY]](http://mashable.com/wp-content/uploads/2012/03/125,pinterest-logo-600.jpg)
Pinterest is now the number three most-popular social network in the U.S., behind Facebook and Twitter, according to Experian Hitwise. A new report from the researcher shows Pinterest got 21.5 million visits during the week ending Jan. 28, a nearly 30-fold increase over a comparable week in Ju…