Having fielded a couple individual emails posing this question already, I thought an actual blog entry might provide a better answer. This is a quick-and-dirty answer, though, so I encourage further discussion in the comments and in class tomorrow and/or Thursday.
1. I don't have a Facebook account. There are a host of reasons why (disillusionment, privacy, security, Zuckerberg).
2. Class Facebook groups have been tried (and failed) in the past. Again, a host of reasons why (bad implementation, creepy treehouse, student disengagement).
3. In my view, college-level courses should involve some level of new challenges. If everyone's already on Facebook, where's the challenge and learning?
This need not be the last word on it, though. I welcome any/all comments here and, if need be, we can discuss it in even more detail face-to-face.