According to Robert Young from GigaOM, social networks will evolve through their communications layer. A good example of that is the MySpace “wall” where friends can post comments about the profiled individual/entity. “… the foresight to repurpose a simple bulletin board to enable communications among a social network of people, all centered on the profiled individual him/herself, proved to be brilliant. More than any other utility for self-expression, it is the wall that keeps members coming back over and over again, often several times a day (to check for new messages). In short, the wall is to social networks what free email proved to be for portals. It is the wall that will also prove to be the most extensible component of MySpace into the future.”
What it means: not only must you enable user-generated content in your sites. You have to allow eventually communications between the participants to truly create a marketplace. Text, video, audio, instant messenging, e-mail, click2talk, etc. can all be used.