I'm member of many different social networks (Classroom 2.0 on Ning, LinkedIN, Twitter, Facebook, and other communities like Drupal.org and Moodle.org among others). Social networks, like other mediums of media, are successful that more people view or participate in them. In large part they are judged successful by how capable they are of connecting you with people or things that you are interested in. Likewise, they tend to fail when they are unable to help you make those connections.