Alex Hern, writing for The Guardian:

In a blog post, Facebook’s product management director, David Baser, wrote that the company tracked users and non-users across websites and apps for three main reasons: providing services directly, securing the company’s own site, and “improving our products and services”.

Most people are aware that Facebook is actively using the data you explicitly give it by posting there to build an advertising profile about you to allow ad targetting. What significantly fewer realise, in my experience, is the degree to which the company is mining both the whole web — through Like buttons — and also building "shadow profiles" through data acquired from other sources.

I'm not sure that the mainstream reporting around Facebook and data has done enough to make that clear — yet.