Facebook.com has apparently added a new "beacon" feature to track its users activity on external websites.
"The second aspect to this new ad system is the expansion of the Facebook data collection and reporting process beyond the confines of the Facebook platform. This is where the "Beacon" concept comes in — as participating sites will add a few lines of code to their website, acting as extensions of the Facebook surveillance system."
"For example, when you engage in consumer activity at a partner website, such as Amazon, eBay, or the New York Times, not only will Facebook record that activity, but your Facebook connections will also be informed of your purchases or actions."