NYC Educator says that it’s not even close to worth the risk for teachers to friend their students on Facebook:
I don't friend students on Facebook, and I'd advise you not to do so either. Simply having done that could potentially bolster a false claim against you. Now that there's no rubber room, I suppose they send you to file papers somewhere. But in the case of a sexual accusation, you could be sitting home without pay or health insurance. To me, it's not worth the risk. Also, I don't really want to know what my students do on Facebook. That's their business. Mine is making them learn English.