The whole reason for educators to fear Facebook was because "it's unsafe and can't be monitored." Which is partially true, but wasn't convincing enough for me.
Now with 1:1 programs popping up all over, companies like Edmodo, Moodle, and Schoology are developing safe ways for teachers and students to communicate outside of class time. These free online tools are great, but they operate on a more closed basis than Facebook.
However, I don't run my class like that. My class is open, ideas are shared, and if someone goes along the lines of inappropriate, then that's just another opportunity for me to teach.