The Pew Internet & American Life Project reports that "the share of adult internet users who have a profile on an online social network site has more than quadrupled in the past four years -- from 8% in 2005 to 35% in 2009." Many people use social networks for personal rather than professional reasons.
See below for a Pew Internet presentation on how social network demographics have changed over time.
The Democratization of Online Social Networks