I recently wrote a blog post about safe and free ways to use social networking within the classroom.
Social networking is a GREAT tool for the classroom (I have some lesson suggestions for various grade levels on the blog), but it can also be tricky. Some sites, like Facebook, might not be appropriate to use in schools, plus you have to be 13 to start a Facebook account. But there are a TON of social networking tools available for teachers, and they really do make teaching more meaningful (and fun!).
Thomas, this is a great question, especially considering that Social Networking has become both a positive and negative part of schools. Kiwi Commons specializes in education parents and teachers about the Internet and we have an entire section of resources based on Social Media. You can check it out here. I hope those help and please let me know if you would like any further information on the subject.