At my school the coaches use Facebook to send announcements about practice times, changes in game schedules, and so on. It's fast, and the information seems to get to students better than more traditional messaging means.
I personally use Facebook a lot for personal networking, but I will not "friend" students, because the kids don't know how to edit themselves. Inevitably they post things that I just did not need or want to know. I also am very circumspect in what I post about myself.
I do let students know that I have a Facebook account. Although it is set up on high privacy so that only approved (adult) friends can see my information, status updates, and photos, anyone can send me a private message through it. Students do take advantage of this feature to make contact with me when they are absent or need to get in touch quickly. It's easy, since there's no email address to remember, and they can reach me anywhere, since I have the private messages set to go to my cell phone. So far I have not had any issues with doing this.
I think it's folly for educators to hide from social networking. It's very powerful technology that is not going to go away. Better to teach kids to use it productively now than send them out uneducated on the matter.