Like Jeremy, I struggle somewhat with how to comment on public FB posts in a sensitive manner. For the most part, I've been pulling general suggestions from my readings of all the posts and then making a global comment to everyone. When a post is particularly skimpy or undeveloped, I've been sending a private message to the student, indicating that s/he needs to add more detail, explain more fully, etc. This seems to be working for now.
On another positive note: students have been communicating with me much more frequently outside of class now that I'm using FB. I've always given them my full contact info but more of them ask me questions via FB than they ever did using e-mail or voicemail.