Why is it watching you?
Facebook pays close attention to your behavior in order to zero in on your personality and serve you ever more highly focused ads. More importantly, Facebook is still learning how to get you to click on the ads it shows you. The rate of click-through on ads displayed on Facebook is lower than on Myspace and light years away from Google. To continue growing, Facebook needs to know what makes you click.
Beyond that, the philosophy that drives Facebook requires an ever-expanding base of information about every one of its users. The goal is to make your profile indistinguishable from your actual self, a core part of your identity in the same way as your voice or fingerprints. The more closely intertwined you are with your profile, the closer to the center of your entire web experience your profile becomes.
How many websites do you frequent that already allow you to connect with Facebook in some way, either by sharing their content on your page, liking their content, or permanently linking your behavior on their site to your Facebook profile? Those linkages do more than allow Facebook and its partners to share your private information. They make Facebook the beginning and end of every trip you take on the Information Superhighway.