I have two teenagers, and thankfully they do not spend every waking hour on it. My niece on the other hand, has been the brunt of some "cyber-bullying" which escalated to a fist fight at school. She was grounded from Facebook for awhile. My thinking is we as parents need to keep a better eye on what goes on with them. I probably spend more time on it than they do, but I will go to their pages once in awhile just to see what has been going on.
Kids need to be taught some ground rules regarding using Facebook as well, and if they can't follow them...no more account! I know it is easier said than done, but if they are made aware of the dangers they will have a respect for it. As far as parents and teachers believing comments posted on Facebook; they should know better. They are just as guilty if they can't get to the truth the old fashioned way...talking face to face with their child about what is happening.