A while back, I did a posting about 35 Social Networking Programs Other Than Facebook and Twitter. The program that still stands out as the best is Edmodo, which recently upgraded its appearance. It now looks even more like Facebook, and your students will enjoy using it for that reason alone. Edmodo offers you and your class a private social network, where you can have class discussions or message students privately.
I love Edmodo's version of the "like" button in Facebook:
If you haven't looked at Edmodo, you're missing a really wonderful free program.