Facebook, since its mistakes with the Beacon advertising incident, has rolled out one of the most robust security systems for any social network, which allows users to control who sees what information about them with great specificity. Take a look at Facebook's privacy features and how to set yours.
Customize Your Social Networks
"Customize" lets you choose which of your Facebook networks can see your profile. Maybe you are comfortable sharing your information with your company softball team, but you aren't anxious to give the same kind of access to thousands of people in your company's Facebook network. Click "Customize" from the drop down menu; a pop-up menu allows you to select some networks to view your profile (like the softball team) while preventing people from other networks (like work) to see it without friending you.