To be honest, while yeah there may be some unwritten code for fans, they're not such a douchebag in that they set out to spoil it for you. Facebook and twitter are just like blogs for people, they wish to say something about something they'll say it, and in no way do you have any right in stopping them, so it's perfectly fine to talk about spoilers on their page.
And it's true they should have shown a bit more restraint, however it's not just their fault, you for starters should have taken this into consideration, it's a major episode being the finale and people find it hard to contain such excitement over it, also considering you follow TVN subs that aren't released for a few days you can't expect people who chose to watch the first available sub to pander to your needs. It's your own choice that you decided to watch a later sub and so should have considered the risk of being spoiled and thus avoiding social websites for a little while.
Hell that's how I avoid spoilers irl, I tell people to shush and I avoid places where people are prone to posting spoilers.