You're Not Sharing What You Think You Are on Facebook:
The researchers surveyed 65 Columbia students on what kinds of information they wanted to share on Facebook, and to whom. For example, many said they were comfortable sharing information about their sexuality to their Facebook friends but wanted to hide it from strangers. Ditto, information about their alcohol consumption. Researchers then determined what information the participants were actually sharing, and how that matched with their intentions.
It didn't match at all: 93% of them were sharing something they wanted to hide, while 84.6% were hiding something they wanted to share. In fact every single participant's privacy settings were not in line with what they actually wanted to share.
This wasn't simply a problem of negligence. The students cared strongly about what they were sharing and believed they had set their privacy settings to reflect their concerns. Just like you probably do.
The problem is that the way Facebook handles privacy doesn't reflect how people view their privacy in the real world. Facebook organizes its privacy settings through type of information. So you can infinitely customize who can see your wall posts, status updates, photos, etc. But what people really care about is this information's content in the context of our relationships, not its form.