I've been grappling with this: I'd love to claim a small fraction of the hours a day I know many students spend on Facebook, and be able to communicate with them through it. Kids, at least at my school, don't so much email. They Facebook and text. However, I would be uncomfortable friending students with my personal Facebook account. It's private space where I interact with my family and friends, and it needs to be separate from my professional life.
I don't know why I didn't think of this before, but over the past few days I'm liking the idea of a separate Facebook account for Miss Nowak, not for Kate Nowak, that I can use to friend students. Not that I would require it - some students don't even have a Facebook. But if they do, they will get updates from me like "Just posted grades from Trig unit test", and I can share interesting and useful links with them where they are most likely to see it. I also have to investigate the usefulness of creating Facebook "pages" or "groups" for classes, but I don't know much about them yet.
If you are using Facebook with your students, please let me know how you have it set up and how it's working for you.