You don't just socialize on Facebook. I listen to music on Spotify (tied up with Facebook), I read articles people post on Facebook, and most importantly, I vis

You don't just socialize on Facebook. I listen to music on Spotify (tied up with Facebook), I read articles people post on Facebook, and most importantly, I visit hundreds of web pages with Facebook "Like" buttons which Facebook can use to build a profile of what I'm like even when I'm not on their site.