I am a sporadic user of LinkedIn and Facebook, plus I have briefly looked at social networks based on Ning. And here is the thing - I cannot seem to figure out, what is it exactly that these “social network” systems are doing that a regular forum isn’t.
E.g. let’s take the enormously popular phpBB. We could setup a phpBB installation, have a separate thread with “user profiles”, a separate forum full of subforums aka “groups”, another subforum for news announcements etc.
In other words, it seems to me that with proper administration/planning you can build any run-off-the-mill online community without investing in renting or customizing Ning.
Perhaps one thing that we WOULDN’T have here would be the “friends” relationships between users. For one thing, yeah, so, is it such a big deal? For another, it could be implemented as a separate web application that would operate as a bot inside the main forum, e.g. updating your friends info on some personal workspace of yours.