Since Facebook won’t provide you a way to opt-out of Beacon, their intrusive advertising platform, here’s how you can prevent them from harvesting data about you from other sites.

For Safari, first install the excellent PithHelmet plugin that lets you block sites, pages, cookies, and advertising with excellent granularity. It is not free, but well worth the $10. Go to PithHelmet preferences, right within Safari’s preference dialog.

In the next step, you’ll need the following Matching Pattern. Here it is for copy/paste convenience:
.*facebook\.com\/beacon.*

For Firefox, I refer you to Nate Weiner’s original post that inspired mine for Safari. He also has a detailed analysis of why Beacon is a privacy nightmare with no means for users to opt-out completely. The ‘No Thanks’ button only prevents the information from being displayed, but Facebook still logs and preserves it, and may do with it as they please (according to their terms of use).