I agree with this, what are teachers doing contacting students on a social site? To me, that's like them hanging out with them in the treehouse....SO not appropriate.
Now if they communicated with the parents via facebook, that's a good thing. As Psmerf said...why wouldn't they communicate with the parents?
With all that being said however if there was a site dedicated to the class whereby the teacher could post information or communications in regards to homework, associated articles that would help with the class agenda, notices of upcoming tests etc, that's a good thing. Same as if the student had questions that couldn't be addressed during regular class hours (like if they were doing homework and didn't understand something) But communication with a student socially is SO inappropriate.....who is monitoring this communication? NO ONE and that is wrong on so many levels.....
Maybe Zuckerberg should create a new "Classbook" where each class gets a page, the teacher monitors what's being posted or said, and the principal has automatic oversight to the whole thing including private messages. That way no matter what is posted or "said", there's a record. It would be relatively easy to set up a filter that would flag any messages that contain any words like dick, penis, tits, boobs, breasts, tatas etc. This filter would automatically kick a copy of the messages to the one policing the "book".