"Facebook's new ad service, launched earlier this month, gets around the traditional third-party advertising cookies by doing the tracking on its own. When a person visits a website selling shoes on a work PC, a piece of Facebook code placed on that site—Facebook's own cookie—recognizes that the person has logged into Facebook using that browser before."

How does this even work? Is the shoe seller placing some kind of JavaScript snippet on their website in order to allow Facebook to track users? Is this coming from their "Like" button?