Ning

Ning
http://ning.com/
...an easy way to create your own social network by allowing you to create the profile questions, upload all types of media, and more; use a private social network to create an interest inventory for the students in your class!

Heaps of Private networks to choose from (or create your own). An interesting one that had relevance to me as a teacher was the classroom2.0 room @ http://classroom20.ning.com/ as it says "the social networking site devoted to those interested in the practical application of computer technology (especially Web 2.0) in the classroom and in their own professional development. "

Have a look, tell me what you think.
Andrew