Some peope (well, in my age group) actually reveal quite a lot about themselves on FB. It's amazing what information they'll post...
That aside, you can read a lot into what people have written, which is what makes it predominantly bad. Sometimes it's a good thing. A friend of mine was worried her friend was suffering from Depression but it was dismissed as 'teen angst.' This girl admitted through private messages to my friend she was being abused by her father (hence the dismissal I'm guessing) friend went straight to police and her mother.
Just like MN, some people use Facebook as a way to communicate things they don't want to discuss in 'real life.' Oddly, a lot of people I know will only have Deep and Meaningful discussions with very close mates over technology. 