From the Ning website:
"What is Ning? Ning is a platform for creating your own social network. Our passion is putting new social networks in the hands of anyone with a good idea. With Ning, your social network can be for anything and anyone. You start by naming your social network and choosing a combination of features (photos, videos, forums, events, etc.) from an ever-growing list of options. You can then customize your social network's appearance and launch it! People who join your social network will automatically have a customizable profile page and will be able to message and friend each other."
While you aren't required to CREATE your own Ning, you might want to spend some time looking at several Nings that were created by folks at the University of Illinois at Springfield:
UIS Community of Practice for E-Learning - http://uiscopel.ning.com/
New Century Learning Consortium (NCLC) - http://nclc-online.ning.com/
Social Networking: Twelve Top Tools You Can Use in Class Tomorrow! - http://socialnetworkinginclass.ning.com/