I don't have much hope for ning. Social networks are pretty easy to program yourself, and their appeal is largely the userbase. You join a social network because all of your friends already did.
If I wanted social networking for my little niche, I'd simply start a facebook group.
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.
I used to have sort of the same issue with Ning as I did with pre-launch Xobni, which is that descriptions failed to make clear exactly what the prodcut does. They're both pretty clear now, but you're right that for most people, there doesn't seem to be much reason to use Ning.
I could see it gaining some traction with corporate intranets, though, especially with a focus on content sharing.
If I wanted social networking for my little niche, I'd simply start a facebook group.