17% of the time Americans spend online is on social networking and blogging sites

"The Nielsen Company today reported that time spent on social network and blogging sites accounted for 17 percent of all time spent on the Internet in August 2009, nearly triple the percentage of time spent on the sector a year ago."
Note - this excludes video sharing sites like YouTube, which are effectively social sites, so the total percentage of time spent on social media sites would be much higher
Source: The Nielsen Company, as reported in a press release, 23rd September 2009