eMarketer estimates that 80m web users (41% of the US internet population) visit social network websites at least once each month - an annual increase of 11%. The firm forecasts that 52% of American internet users will be regular social network visitors by 2013. In addition, as membership levels rise, users are also becoming more active. eMarketer's study predicts that 79.9m people (40% of all US users) will generate content on social networks at least once per month during 2009, either by sharing content online or communicating publicly - equivalent to 90.5% of all social network users.
eMarketer: http://www.emarketer.com, 09/02/2009