I attended the 50 Ways You Could be Using Facebook in Your Classroom presentation at FSI 2010.  About 18 people present. Why Facebook is so good:

• Students already use it daily
• Levels the playing field; it's their environment, not yours
• Invites 2-way street of conversation

Tips:

• Create a teacher/school profile (a separate account)
• Invite people to "like" the group (rather than "friending" you)
• Setup multiple groups for each course, group, etc.
• Explore lots of apps; a few worth noting:
  • Calendar (30 boxes)
  • CourseFeed
  • SlideShare
• Manage privacy settings very closely
• Tag students to help get the message out (also shows content to "2nd degree" circle -- a.k.a. free marketing)
• Bottom line: reach out and communicate with students