People who complain about the lack of privacy on Facebook are often just misguided as to what Facebook is. Facebook is by it’s nature a public medium, unfortunately people don’t seem to understand this. Yes, you can choose who your friends are (to an extent), but with those selected people you share anything you do on Facebook and an apparent lack of privacy only becomes an issue when you forget this.
People don’t seem to get the fact that you are not Facebook’s customers, you are it’s product. Your information is what it sells to keep itself alive. It needs you to make money, but it’s rewarding you (call this payment if you will) by allowing you to use its service without paying a penny. If you want complete privacy then you may as well leave the internet now, everyone here collects data, because that data is valuable to a multitude of people in terms of advertising and figuring out generally what you want. Which is important, because when the machine knows what you want, it can make more of it!
If you don’t want your friends knowing you’re against abortion don’t join the group. Simple as 
Google works on what is essentially the same principle. I understand that the company datamines my email, my voicemail (I use Voice), my searches, my website’s content, my documents and my instant messages to provide advertising targeted specifically at me. It is, as a business model, incredibly effective.
However, Google, doesn’t give out my personal information to the advertisers; doesn’t give out my information to websites I connect to; doesn’t leave a publicly accessible profile with my personal information unless I opt into it (Buzz problems aside).
For me, maybe the matter is more a case of principle. I understand datamining as a very common tool in the public (government) and private sectors. My personal opinion on privacy is that we need to change our viewpoint: If you don’t want it made public, don’t put it online. I am personally apathetic about most information about me on the internet, but I can understand the concerns of many other persons online. Facebook draws my own personal ire because they have reneged on a series of highly-public promises not to do precisely what they are doing now, and they are doing it for nothing more than to make a quick buck.
Facebook changes its privacy policy at least once a year since it was created http://www.eff.org/deeplinks/2010/04/facebook-timeline/