Now that Mark Zuckerberg has seen the evil of Facebook's ways, you can now opt-out of Beacon's spying (as far as we know.) The process is simple, despite it not being so obvious.
- Log in to Facebook.com
- Click on "privacy" in the top right
- You will now be on your "Privacy Overview" page. Click on "External Websites" or the "Edit Settings" link to its right

- Check the checkbox and hit "Save"
You know you're relatively* free from auto-Beaconing when the image next to Edit Settings changes from this
to this
Finally, it's a good idea to log out of Facebook when you're done with it. That's a good rule for any site that you don't want prying eyes on.
* Note that on the External Websites profile page, Facebook warns you that,
Please note that these settings only affect notifications on Facebook. You will still be notified on affiliate websites when they send stories to Facebook. You will be able to decline individual stories at that time.
FYI, Beacon still creeps me out.