I have a Facebook page. It's very clean and work safe because...
(1) employers can see it
and
(2) my students can see it.
There was an article on cnn.com the other day about how a lot of people believe students and teachers should not be "Facebook friends." While it's true that there are *some* sickos out there who got a teaching job just so that they could do horrific things to minors, I find that being Facebook friends with a lot of my inner city students helps me to have an open line of communication with them. They get to know me as a person a little bit better by seeing my likes and dislikes and interests; it can be hard for children in poverty to trust "authority figures," so this allows them to learn more about me and learn to trust me so that I can mentor them.
With Facebook, I can keep track of my students over the summer and follow them off to college; a lot of them don't have an adult in their lives who is there to ask the questions they have about getting ahead in life. Any conversation I have ever had with a student on Facebook is something I wouldn't mind sharing with a colleague or that student's parents.
For me, Facebook is not only a social networking tool, but also a teaching tool.
Even our school principal has Facebook, and he's friends with tons of kids. He's always posting lots of educational and motivational things on his Facebook page.