Facebook doesn't tell users everything it really knows about them
Facebook's page explaining "what influences the ads you see" says the company gets the information about its users "from a few different sources."
What the page doesn't say is that those sources include detailed dossiers obtained from commercial data brokers about users' offline lives. Nor does Facebook show users any of the often remarkably detailed information it gets from those brokers.
-- Julia Angwin, ProPublica.