FYI I started up a new VSN on Ning called GeekBuddy. You can find it at geekbuddy.ning.com. I built it mostly to learn more about the Ning platform, since I think it'll be a viable candidate for people & organizations who want a straightforward, attractive, hosted social network. You can even request the source code (as I have) & make direct modifications. I'll post more thoughts on Ning as I work with it more, but for the time being, if you want to join the GeekBuddy network and say hello, that would be awesome. :)

There are at least two things about Ning that make it not right for everyone, though:

1. It's a hosted solution, which means you don't have full control over performance, uptime, and upgrade schedule.
2. You can't 100% remove all Ning.com branding (although you can remove a lot of it). So you don't have a fully-encapsulated user experience, and people will know you used Ning to make your site (which isn't necessarily bad).

For point #2, I think some people worry competitors will see that they used Ning & quickly crank out a clone. I don't think it's as much of an issue as one would think -- for example, no one has that concern about putting a forum on their web site, even though forum software is (now) easy to install.

If your competitors are smart, they probably already know about Ning or some other quick SNS solution. And since software is becoming increasingly commoditized, the way you beat your competitors isn't going to be the software -- it'll be things like your capital, partnerships, content, creativity, marketing, and any customizations you've made to the platform.

Of course, it would be better if your competitors didn't find out about how easy it was to make a social network, but anyhow...