Google recently announced their new Social Network, Google+ ('Plus') with several features that distinguish it from Facebook. The most interesting one in terms of classroom use may be the "circles" feature in which you can share content across distinct, separate groups.
The core Google+ features are:
Circles: A method of assigning people to different micro-networks and then sharing information with only them
Hangouts: Video chats where people can get together and catchup
Huddles: Ongoing IM conversations across groups
Sparks: Search for content by your friends and share your own content
Folks are already developing a collaborative Google Doc for tips and tricks with Google +
Read more commentary from PC World on Google+ and privacy.
See also: thoughtful post by Ryan Bretag: He argues that rather than trying to "school-ify" Facebook, Google + could fit the bill for classroom learning commons.