It's safe to say that Facebook has critical mass. The number of businesses that have Facebook pages is massive, Safari's spellchecker thinks it's a word, and it's incredibly common to see advertisements for competitions where "all you have to do is 'Like' us on Facebook!" This would be annoying for those without Facebooks in any circumstance, but the fact that a lot of people don't have Facebooks specifically because of the repeated blatant privacy concerns, the willingness of the company to pander to advertisers over users, the lack of moderation of hate pages even when they're reported, makes it even worse.
But when you really get bitter irony is when social protest movements operate off Facebook. You're complaining about corporations running our lives while excluding people who have issues with a pretty dodgy corporation. Seriously?
It might be convenient, but it also serves to really irritate some of the people on your side. Just saying.