it is a problem - but it's not facebook's problem, it's your problem for not understanding the settings (obviously i do not mean you, i mean users). projects like beacon were facebook's problem, and they were wrong for doing that. if your using a product and dont understand how it works, then you only have yourself to blame. example (dated, i know):
I disagree that it isn't Facebook's problem or that you have only yourself to blame.
Facebook is aware of this problem, and it's good for their bottom line. It's not in their best interest to encourage privacy, so they don't. In fact, they actively work to encourage (or, in some cases, force) more sharing, less private behavior.
As there are serious risks, even dangers, involved in over-sharing (not just your over-sharing but your friends' over-sharing as well), I would say that Facebook has some responsibility to protect its users and inform them on the implications of playing fast and loose with privacy.
"if your using a product and dont understand how it works, then you only have yourself to blame"
Don't we all do this each and every day? And even if you do understand how everything works, predicting the long-term consequences and ramifications can be impossible.
Edit, I found a better example:
http://willmoffat.github.com/FacebookSearch/?q=playing+hooky...