New research released by eMarketer suggests that activity on sites such as Facebook, MocoSpace and foursquare has been a major driver of mobile Internet use.  The firm predicts that the number of mobile users accessing social networks from mobile devices will reach 607.5 million worldwide by 2013, or 43% of global mobile Internet users. In the U.S., 56.2 million people will be social networking via cell phone by then, accounting for 45% of the mobile Web users. eMarketer projects that mobile social network users worldwide will grow more than fivefold in the next five years, jumping from 141.4 million in 2009 to 760.1 million in 2014.  Story at MediaPost.