According to an article in  www.Gigaom.com :

• U.S. web users tripled the amount of time they spent on social networks in August from the same month last year, according to Nielsen.
• Estimated online advertising spending on social networks more than doubled over the same period.
• 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.