One of my old high school teachers has FB, i haven't added him though.
I think the problem is lack of trust, i never trusted my teacher because they all talked to each other about everything, nothing was confidential, so i guess this could help with that.
I see nothing wrong with being friends with your teachers, it makes the enviroment relaxing; which is probably why i enjoyed college a lot more, you could have playful banter with the tutors.