Facebook hit my school when I was a sophomore in college. Like the majority of my classmates, I joined Facebook. I joined Facebook when it was just college students. And then Facebook opened up. And then high school students joined.
I do not friend students who are in my school building. Once you've graduated, if you find me, I might accept your request. Other teachers follow a rule of only friending students who have been graduated for at least five years. It's up to individual teachers to assess what they are comfortable doing.
The problem is closely related to becoming actual friends with your students. If you remove your authority, problems can happen. If you're joking with a kid on Facebook, how can you expect to discipline them in the classroom? Our school released guidelines for communicating with students on Facebook. The main point was to not engage in private messages or attempt to use Facebook as the sole means of contact. After that, it was kind of left up to teachers to decide. I really recommend against friending current students. When you friend current students, it's much harder to stay out of their drama. You also have to remember what you have on your profile. Pictures of your kids might not be an issue. Pictures of you getting your drink on in bikini on the beach? That's not going to be a good idea.
Yes, you can adjust your settings so that certain things are hidden from certain people, but mistakes happen. It only takes one mis-click and pictures of you could be printed up before you know it.
Another reason I won't friend is the same reason that I refuse to read student notes. The less I know about their drama, the better. I don't want to know who's dating who and who likes who and who and on and on. The only reason I need to stay updated on who is dating who is so that I don't sit Tommy next to Sally when they're dating or when they've just broken up. Teenage drama can get quite intense. I've known teachers who have accidentally let themselves get swept up in the sensational nature of it all. Getting caught up in all of that drama isn't fair to any of the parties involved. It's also best to keep yourself from making judgmental decisions on how you maintain your classroom.
And when they ask why you haven't accepted their friend request, you can tell them your policy on only friending graduates.