The Economist has summary of current research into the mood-altering effects of Facebook usage:
the more someone uses Facebook, the less satisfied he is with life.
And here is the root cause, it's envy:
[Researchers] found that the most common emotion aroused by using Facebook is envy. Endlessly comparing themselves with peers who have doctored their photographs, amplified their achievements and plagiarised their bon mots can leave Facebook’s users more than a little green-eyed. Real-life encounters, by contrast, are more WYSIWYG (what you see is what you get).