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Facebook, the top US social networking site since May 2009, attracted roughly 151 million unique US visitors in April 2011, reaching three out of four visitors to the social networking category during the month, according to new data from the comScore Media Metrix.
Read more here
http://www.marketingprofs.com/charts/2011/5212/facebook-growth-outpacing-other-social-media?adref=nl061011
