In an attempt to discover what is so great about Ning, I googled this question, "What is so great about Ning?" and got some answers on the Metafilter website. Ning seems to allow users more ability to focus on a specific topic than Facebook does. One Ning that's mentioned is for zine writers, and the explanation for Why Ning? is that it makes possible a closed group of zine writers, whereas using Facebook would mean that family, and non-zine friends would need to see all this zine-related info that they're not interested in. Another commenter mentioned that a group of opinion piece writers uses a Ning to propose and work on opinion topics in a secure setting; the ideas won't leak out to anyone who shouldn't see them. This Metafilter comments page is from 2009; since then Ning has started charging money for its services, and so is probably much less appealing to many users.
I found some interesting Nings when I searched the topics libraries and school libraries, and I joined one called Library 2.0 This Ning has many members and looked promising, but on closer inspection doesn't seem to have much "action"--in other words, I don't see many recent postings. Could it be that the library world is saturated with opportunities to keep up to date professionally and doesn't really need this one? Right now I follow library news on my Google Reader, on the LM_NET listserv, through a library group on Diigo, etc. At some point a person has to say to herself, "I guess I'm up-to-date on library news!" Maybe that's why the Library 2.0 Ning doesn't have a lot of new news on it; everyone's already covered all the possible topics in other forums.
In our class discussion people wondered why anyone would choose Ning over a blog or wiki, and I am still trying to figure this out myself. The security is nice, but a wiki can be secure if only people with passwords can participate. I'm going to keep searching for answers!