Raganwald describes a Facebook privacy-leak that's creepy even by Facebook standards. When you sign up for apps, the app-maker has the power to extract all your friends' personal info, assuming they've shared it with you. So anything you share with your friends can be hoovered up by any app they trust. If you'd prefer not to do this, there is a setting buried in the Facebook preferences, and Raganwald walks you through checking it off.
Here's an app that purports to help people build their "professional network:"
If you share your work history with friends and they use this app, you've just silently shared your work history with the people who built this app. And your locations data! I have visions of them selling an employee profiling service: "Mr. Braithwaite claimed to be employed with Initech, but he spent an awful lot of time at Sense Appeal Coffee Roasters during that time period…"
… Look at what you're sharing by default with all of your friends' apps! Selfish bastards that we are, we do not wish to make our friends' experiences "better and more social" when they use apps that we don't personally authorize. Turn everything off and save changes. Voila! You've stuck another finger in the dike holding back the endless flood of Facebook privacy loopholes.