«If I’m not paying for Facebook, am I the product? No. Our product is social media – the ability to connect with the people that matter to you, wherever they are in the world. It’s the same with a free search engine, website or newspaper. The core product is reading the news or finding information – and the ads exist to fund that experience.If you’re not selling advertisers my data, what are you giving them? We sell advertisers space on Facebook – much like TV or radio or newspapers do. We don’t sell your information. When an advertiser runs a campaign on Facebook, we share reports about the performance of their ad campaign. We could, for example, tell an advertiser that more men than women responded to their ad, and that most people clicked on the ad from their phone».

Source : Hard Questions: What Information Do Facebook Advertisers Know About Me? | Facebook Newsroom