When you see an ad on Facebook, it’s likely that it’s being served to you because of something you wrote in your profile. But what if you didn’t fill out your favorite movies, your hometown or the quote that inspires you? Facebook has an app for that.
The system, which they refer to as inferential, assumes that a good portion of your Facebook friends are into the same things you are. So an ad that the systems deems appropriate for you, would also be served to your friends.