The Pew Research Center's Internet & American Life Project has released 65% of Online Adults Use Social Networking Sites.
Here's an excerpt:
Two-thirds of adult internet users (65%) now say they use a social networking site like MySpace, Facebook or LinkedIn, up from 61% one year ago. That's more than double the percentage that reported social networking site usage in 2008 (29%). And for the first time in Pew Internet surveys it means that half of all adults (50%) use social networking sites. The pace with which new users have flocked to social networking sites has been staggering; when we first asked about social networking sites in February of 2005, just 8% of internet users — or 5% of all adults — said they used them.