I actually think Ning is a clever idea, but I don't see it as anything more than the social networking equivalent of Eve (commercial message board software) or Topica (commercial listserve software). If well-trafficked websites are willing to pay for third-party forum software, why should they not be willing to pay for social networking software?

I don't think Ning is expecting people to try and build the next Facebook on their platform... they're just making it easy for pre-existing communities to add social networks.