In my library project, Thing 26 is "Join the 23 Things Ning".
I joined just now in round 2 because I didn't participate in round 1. I uploaded my photo. I sent messages to two members; one is my new co-worker and one is someone I knew in my community years ago. I also joined my library system's group.
As far as seeing uses for a Ning, I imagine it's useful for networking and for discussing ideas more easily with people outside of your own library. It would depend on how much activity the Ning sees; the more often people are on it having discussions, the more value it would have. Otherwise you'd be putting a question out there and getting silence in response. It could work kind of like a meeting you could access anytime you had something to ask or share. I was really hoping it would be a way to get back in touch with people I've worked with in libraries years ago, but there are so few real photos and not even that many real names that I didn't find many people I know.
The only other Ning I've ever seen was for an alumni group, so I'm not sure I understand how a Ning could work for me in my personal life. If they work basically like discussion boards on the Internet, just a grouping of people discussing a topic, then I can see lots of potential uses for them; they would be a way to facilitate communication for any group.