I have mentioned Facebook and the expectation of privacy before. Now, the blog of the Canadian privacy commissioner is highlighting one of the risks. Because third party applications have access to both the data of those who install them and the friends of those who have them installed, they can be used to surreptitiously collect information from those in the latter group. While this widens the scope of what third party applications can do, it also seriously undermines the much-trumpeted new privacy features in the Facebook platform.
It just goes to reinforce what I said before: you should expect that anything you post on Facebook is (a) accessible to anyone who wants to see it and (b) likely to remain available online indefinitely. The same goes for most information that is published somewhere online, including on servers you operate yourself.