chknptpie makes some good points. Some people have come to rely on Facebook a lot, kind of like some people rely on MSN. They enter their phone number, full name, family members, etc on the site without thinking twice about it. People are no longer as careful about that as they once were, which is both good and bad. The internet is becoming more personalized, and that means that your info is being entered into several database, available for all hackers.
When faced with the fact that Facebook is NOT as reliable as they thought it was (the recent chat bug comes to mind) it's kind of a wake-up call. Facebook has become more than a social networking site - it's a meeting place for people.
But think about this - someone writes on their Facebook "Yaaaay, I finally have my vacation. Hawaii for two weeks, here I come!" And on that same site, you have your name and your phone number. And EVERYONE can see this. It's an open door for burglars to come and steal all your stuff!
But if the default settings of your account were set to private, so only your closest friends could see it, then you would have nothing to worry about.