Okay, I don't think I should have to explain this, but understand that Anonymous is not a group of super heroes out for the good of all. Encyclopedia Dramatica covers it fairly well:
http://encyclopediadramatica.com/Anonymous
While Anonymous does a lot of good things (from my perspective, and that if the general internet userbase) they also do a lot of bad things just for fun. Or just stuff that makes people go "WTF" without being good or bad.
I don't follow them too often, nor do I frequent 4chan or other sites/area where they hang out. But don't forget they also use chat rooms and such (going by logs) as well as message and image boards, and whatever else they need. Just because you scan /B/ quickly doesn't mean your going to find answers to anything they do. What's more there definatly seems to be a distinction between the people involved, and I haven't been interested enough so far to really find out how all the politics work and what sites/groups tend to get what done.
The point here being that there might not be some great and noble reason for messing with Facebook. It might just be that the guy running it is apparently a douche (and is getting a lot of attention on that account due to a recent movie, hence the timing), and has decided to try and make himself seem like less of a greedy, backstabing, thieving dork (according to articles and apparently that movie which I have not seen) by donating a hundred million dollars to education. Messing with his site might be a way of saying " we still think your a dork despite the charity". Or maybe they are just doing it for the lulz, the fact that we're talking about it now being the entire point. Unless Anonymous clearly states a purpose it's generally hard to say what their motivations are at any given moment.
I will also say that from what I've seen, the people in Anonymous happen to be a mixed bag. While a bunch of people from /B/ acting in concert as a raid can do a lot of things, there seem to be a core of people that have some real skills that can do incredible amounts of electronic manipulation, and their involvement is what makes the differance between an epic action, and something that is kind of a joke.
To some extent I tend to think of Anonymous as sort of being a descendant of the old "Legion Of Doom" (hacker group, look them up along with MoD on say Wikipedia). Albiet a lot more active. What guys like Lex Luthor did back in the day were fairly epic.
At any rate all rambling aside, it seems to be over, time will tell if anything is going to come of it.