In case you’ve been sleeping, and don’t know what Facebook Beacon is:
Beacon is a part of Facebook’s Ads system that sends data from external websites to Facebook, ostensibly for the purpose of allowing targeted advertisements, and allowing users to share their activities with their friends. Beacon was launched on November 6, 2007 with forty-four partner websites.
Here’s a very short Jingcast of how Facebook may let you opt out of this service.
Note: “It is as yet unclear whether or not opting out of the Beacon program stops third party websites from collecting information from Facebook entirely.” (Source)