That's the crux of the problem - that most people's mental model of what's public and private doesn't fit with the way that Facebook is currently arranged. Where there's dissonance people are going to get caught out, embarrassed, outed, sacked, or worse depending upon what sort of regime you're living under. Privacy breeches can be trivial or humorous, but sometimes they can also have much more serious consequences.
If you're a technologist who has followed the development of the web closely, and I suspect that most people on this site fall into that category, then you have a much better understanding of what the risks are when putting information about yourself online, but most ordinary Facebook users aren't technologists or hackers and don't have the same understanding.
It seems some of the culture around what newbies should do has also shifted, making it more likely that people will end up with their data somewhere they didn't expect.
It used to be that, even if they knew almost nothing about the internet, one of the first things hammered into a new AOLer was: you should be careful about putting personally identifiable information on the internet, at least until you understand exactly what you're doing. The norms for things like mailing lists and web forums were to use psuedonyms, etc. I think I was even taught that in a computer-literacy class in school at some point. So it ended up being sort of conceptually opt-in: don't put anything out there until you know what you're doing. That reduces the risk that a n00b will end up putting personally identifiable information into a system that they don't understand, that will use it in ways they didn't expect.
Facebook obviously sends the opposite message: put stuff out there by default, because that's how things work these days.
If you're a technologist who has followed the development of the web closely, and I suspect that most people on this site fall into that category, then you have a much better understanding of what the risks are when putting information about yourself online, but most ordinary Facebook users aren't technologists or hackers and don't have the same understanding.