It's getting more and more difficult to know what anyone does with your data, so I'm not suggesting that Facebook is any worse than other networking site, or the phone companies etc. A lot of the problem with Facebook seems to be that things change quite a lot; for instance, at one point they changed their 'privacy' policy to a 'data use' policy. This didn't really mean any change in what they actually did with the data, but it worried people. Then there has lately been a big story about employers asking workers to hand over their Facebook passwords - which seems appalling to me, but the US House of Representatives have refused to make it illegal. However, this isn't actually Facebook's fault, though it does show the potential for misuse of the site.

So without necessarily blaming Facebook or any other site, I would be worried because the situation keeps changing so fast, few people are able to keep pace with it, and the potential for private information getting into the wrong hands is growing all the time.