Facebook is still a somewhat controversial social networking site. There are some schools, I have heard, that do not allow teachers to have accounts. Some allow it, but do not allow teachers to have students as 'friends.' Ours does not have a policy, but my own policy is to not have students under 18 as part of my Facebook, unless they are relatives. I think that there are times other people may post inappropriate things I do not want children to see, and it is my personal site.
Saying that, I think it can be a valuable tool for parents. Last year was the first time I was on Facebook. I joined for my own enjoyment, but soon found many of the parents of my students wanting to be friends. I have often thought I would like to created an account just for our class. This would be another tool that would allow communication, and in some cases, instant feedback and responses.