How Consumers Interact with Brands on Social Networks - eMarketer
http://www.emarketer.com/Article.aspx?R=1007742

Coupons remain a leading driver of brand interactions in social networks

“Those who still think that social network users are too busy engaging with friends to notice marketers must change their viewpoint,” said Debra Aho Williamson, eMarketer senior analyst and author of the new report “Brand Interactions on Social Networks.” “Brand interactions are real, valuable and growing. “ According to a February 2010 survey by Chadwick Martin Bailey, a market research firm, 33% of Facebook users have become fans of brands on the network.