Scientists at Facebook have published a paper showing that they manipulated the content seen by more than 600,000 users in an attempt to determine whether this would affect their emotional state. The paper, “Experimental evidence of massive-scale emotional contagion through social networks,” was published in The Proceedings Of The National Academy Of Sciences.
This story is incredible and should be the only reason you need to immediately leave Facebook. It's just not worth it.
Facebook is essentially telling people that they care so little about users that they see no reason to refrain from directly trying to control users emotional state. You'd be pissed if anyone tried to manipulate you in this unapologetic way, so don't let Facebook get away with it.