A blog post in Marketing Pilgrim says that two-thirds of marketers surveyed said they were using SMM, up from only 20% of marketers in the 2007 survey. The most popular social sites for marketers were Facebook (74%), YouTube (65%), Twitter (63%), and LinkedIn (60%). Only a quarter of marketers were using viral video in 2007.
http://www.marketingpilgrim.com/2009/08/viral-video-social-media-marketing-up.html