It's always disturbing to realize how much of what we consider to be private actually isn't when it comes to social networking. In fact, when I returned to the library I saw that on the front page of the NY Times today is an article titled "How Privacy Can Vanish Online, a Bit at a Time". It is all about how researchers were able to comb through Facebook pages for example and glean tons of personal information about people, more than you would expect even with privacy settings. Definitely read the article here.
The bottom line is that the internet is public - no matter what website you are on. The article ends with a quote from Cornell University computer science professor John Kleinberg, "When you're doing stuff online, you should behave as if you're doing it in public - because increasingly, it is." Well said.