One thing which could be quite useful is Jay’s Tips for people who are concerned about joining Facebook. Though he wrote those three tips for his students, they could apply more widely. They’re quite straightforward and sensical. (Which shouldn’t be surprising as Jay’s in math at Harvard. If he were to not make sense, the world might collapse.) Summarised (from Jay’s already brief tips): use privacy settings, think about using a pseudonym, get a friend to register for you.
Personally, I’d say that it’s probably best to heed the first of the three tips. While Fb does encourage members to post all sorts of potentially sensitive information, it’s good practise to carefully treat any information you may provide online. Despite the ongoing media coverage on privacy concerns on Facebook and elsewhere, the main point here is that there are varying degrees of privacy which can be applied to information distributed on- or offline.
There’s a lot more to say about learning/teaching uses of Fb.
One thing I like about Fb in educational contexts is that it encourages a type of candour or, at least, some amount of transparency. Public information about members of a class (registered students, instructors, assistants, auditors…) can be very helpful as a course progresses. In fact, I’ve been thinking quite a bit about Fb-like features in Moodle, such as elaborate profiles, ability to build links across courses, ad hoc groups, etc. Moodle and Facebook share several features and there could be a rich integration of features from both.