This WSJ article reports on a new privacy issue with Facebook. It turns out that their application infrastructure allow those applications access to your personal information independent of your privacy settings. They are then able to (and have in many cases been shown to) share that information with third parties. The specific information shared is your Facebook user ID, and in some cases your friend’s user IDs.
Many of the most popular applications have been shown to be sharing this information, including FarmVille, Texas HoldEm Poker and FrontierVille among others.