Somini Sengupta for the New York Times:

In Facebook parlance, it was a sponsored story, a potentially lucrative tool that turns a Facebook user’s affinity for something into an ad delivered to his friends.
Amazon is one of many companies that pay Facebook to generate these automated ads when a user clicks to “like” their brands or references them in some other way. Facebook users agree to participate in the ads halfway through the site’s 4,000-word terms of service, which they consent to when they sign up.

This is an interesting reminder that when you get to use a service like Facebook for free, you are, in fact, the product that’s being sold – whether you like it or not.