People tend to be really fake on Facebook and update with meaningless events in their lives nobody gives a fuck about.
Faux-knowledgeable atheists trying to convert their friends for whatever reason to the point where it seems dogmatic. Funnily enough, I've never encountered anybody actively trying to turn people religious yet the majority of my friends are religious.
People trying REALLY HARD to be funny (using lots of "haha", using a lot of acronyms just in case you weren't sure if he or she was joking of not).
Topical activists who don't know what the fuck they're talking about, post one thing and never think about it again as if it helps.
People who use it to gain attention through song lyrics or bitching about their lives.
People who use it as a means of advertising for whatever they're currently doing in their lives so we can spend money to go see their shitty show or whatever.
On the other hand, Facebook is an invaluable tool to keep in contact with friends so, despite the negatives, I keep mine and just dont use it unless I need to for social reasons.