Facebook is really smart. Not only does it know all the things you tell it (Which is often quite a bit more than you should), but it also seems to know what websites you've been to, and other random information like that. Because it does. Sometimes this seems rather creepy (Okay, it is), but it can be really convenient sometimes. And sometimes it isn't so convenient.
One feature in Facebook is personalized ads. Facebook will advertise to you based on which websites you look up on the computer. Your web browser will download a "cookie" (you know, like the ones the dark side has?), which will help your computer perform faster on that website. Facebook can then use that cookie to recognize that you have been to that website (where else would you get that cookie?), and then presume that you are interested in the things on that website, and then give you advertisements for similar websites. This is usually fine and dandy, but I have recently started getting ads in Korean and Japanese (but not Chinese, for some reason) in excess, and I cannot figure out where I've been that would give me these obscure ads that I can't read.
Another feature of Facebook that I discovered just this morning, is that it will recognize a changed password. It turns out that an old password won't get you into your account no matter how many times you try it. And Facebook will tell you that you are trying an old password. And when you changed your password. At first, I thought that Facebook was just sassing me: "You changed your password at 9:38 last night. Duh." But then I realized that that was a really cool feature. If someone else got onto your account and changed the password without you knowing, you could see that there was a new password, and you weren't just having a typing error.
Probably my favorite feature about Facebook is creeping. You don't need to know a person to know everything about them! Although, depending on security features, a mutual friend helps. Seriously though, I have looked up so many people that I have never met before by using Facebook, and it's a little bit creepy. But oh so useful. Your roommate has a new girlfriend? Facebook stalk. You conveniently saw the name on the top of the paper of that cute girl in your biology class? Facebook stalk. Everyone does it, so of course it is socially acceptable! Right? ....Right?