70% of consumers trust opinions of unknown users. (Econsultancy, July 2009)

This probably shouldn't be a surprise, but reading it phrased like this really made the point stick out (although I'm not sure what to do with it at this point):

It's from Social Commerce Statistics, Case Studies, & Research | Bazaarvoice

Which includes the following stats:

"90% of consumers online trust recommendations from people they know; 70% trust opinions of unknown users. (Econsultancy, July 2009)"

"In a study conducted by social networking site myYearbook, 81 percent of respondents said they'd received advice from friends and followers relating to a product purchase through a social site; 74 percent of those who received such advice found it to be influential in their decision. (Click Z, January 2010)"

"Users put great trust in their social networks. One-half of Beresford respondents said they considered information shared on their networks when making a decision--and the proportion was higher among users ages 18 to 24, at 65%. (eMarketer, October 2009)"

"Friends still play an important role in influencing consumers. Eighty-three percent of online shoppers said they are interested in sharing information about their purchases with people they know, while 74 percent are influenced by the opinions of others in their decision to buy the product in the first place. (Manage Smarter, September 2009) "