Every year, a new crop of students discoveres that I'm on facebook and starts to "friend me". In the past, my policy has always been to say that I'll friend you once you're about to graduate. I am wondering about relaxing this and either just accepting all of the friend requests or requesting only those from students who will never have me in class again (with a nice note to the rest saying that I'll respond after I'll never have you in class again).

 What are your policies about friending students (and why)?