StumbleUpon drives more web traffic to other sites in the US than any other social media site

"For months, Facebook has been the biggest social media site that drives web traffic in the United States. Those days are over– for the moment, at least.
Web discovery engine StumbleUpon is now the biggest traffic driver among social media websites in the US, according to global web analytics service StatCounter. The company unseated Facebook at the top during June 2011, according to the latest StatCounter social media data. StatCounter tracks hits to over 3 million websites, and its social media data is gathered by analyzing every hit referred by a social media site."
Source:  GigaOm, 5th July 2011