cleasto - 8:38 am on Nov 30, 2010 (gmt 0)

Given the social aspect, you might consider tying in some game elements to drive repeat visits and other user actions. If users are getting feedback from your service that they are doing the actions you value, and are getting some sense of status from those actions, they may be more inclined to share generally or more specifically invite their friends to participate. Things such as points and achievements can be effective depending on context.