In conversations I've had recently about public online communities and social networking sites, the point has been made that if a community is moderated (particularly pre-moderated, that is before publication) then it is closer to a “publishing model” and the owners of the community can be seen as responsible for its content and thus legally liable for any problems - to the extent of being sued.
If a space is unmoderated – an open social network for example, then the owner has some defence against being sued for what goes on there as they have no authority over it. Somewhere in between is a community that is moderated after the fact as it were – mainly open, but where moderators do keep an eye out for objectionable content and deal with it to some extent. Here the legal issues are complicated and watertight policies about when and how to intervene are essential.