As reported at GigaOm:

Around 17 percent of all time spent on the web in August in the U.S. was on social networks, up from 6 percent during the same period a year ago, suggesting that sites like Twitter and Facebook have not only grown their audience size, but augmented user engagement. Meanwhile, advertising on social networks rose to $108 million last month from $49 million in August of 2008, an increase of 119 percent.