Social networking accounts for 16% of the time Americans spend online

"Social networking use has doubled since 2007, and it’s all thanks to Facebook, Twitter and an array of other social companies reaching record traffic highs.
comScore’s latest numbers are out, and they paint a familiar story: social networking is on the rise. It’s the rate of growth that’s surprising, though.
According to the web analytics firm, the average online user in the U.S. now spends nearly 16% of his or her time on social networking sites such as Facebook, LinkedIn, Tumblr or Twitter. That’s up from just 8% in July 2007. In the last year alone, social networking use has increased by approximately 25%."

Source:  Data from comScore, reported by Mashable, 16th June 2011