The term has been added to wikipedia! Here is what they said
Social Commerce is a subset of Electronic commerce in which the active participation of customers and their personal relationships are at the forefront. The main element is the involvement of a customer in the marketing of products being sold. e.g. recommendations and comments from customers. This happens for example when customers publish weblogs with their shopping lists. The term was first introduced by David Beisel and then picked up on by Steve Rubel, and originally referred primarily to sites such as Yahoo!’s shoposphere, and Shopit, where the social component is primarily recommendation and review.
However, the term has been expanded to include a variety of collaborative commerce activities, where the social participation may extend beyond recommendation to collaborative purchasing, such as BountyUp, or fundraising (ChipIn, Crowdfunder, Causes on Facebook).
Social commerce can be correlated with Search Engine Optimization as a way to build inbound links and generate user content, all of which are tools to improve a website’s search results on a given search engine such as Google.