While a lot of people talk about privacy or content relevancy issues, which I agree are important, there is a good thread on Reddit discussing why Facebook can be bad for your mental health.

One user writes, "I started to compare my life to others, hating people based off of their status...All it brought me was anger and a lower self-confidence. Almost immediately after deleting it, I felt better. I'm mostly ok with how I live my life and I don't need to compare my life to that of others."

http://www.reddit.com/r/AskReddit/comments/11wm5w/to_those_w...