I've enjoyed being part of several NING sites for nearly 2 years =for 23 Things on a stick, More Things on a Stick, as well as the MEMO Ning and some more general Ning groups. I like the ability to create a Ning site and to make the content whatever you want. I like the ease with which people join Ning groups and interest groups within a Ning. I find, however, that it is one more place to keep track of and to monitor, although I do have updates sent to my email account for the things of most interest.
One can put just about as much effort as you like into your Ning profile and can be as active as you like. Although it is very different from FaceBook, I like Ning more than I care for FB -- too much "noise" for me on FB and I am disturbed by the Terms of Service.
So - happy to be on Ning and enjoy meeting others there; I enjoy the content, and enjoy the social networking aspects of it.