Facebook tracks everyone that comes across a page with a like button. They also purchase data from 3rd parties https://www.engadget.com/2016/12/30/facebook-buy

Facebook tracks everyone that comes across a page with a like button. They also purchase data from 3rd parties https://www.engadget.com/2016/12/30/facebook-buys-data-on-users-offline-habits-for-better-ads/ https://www.engadget.com/2016/12/30/facebook-buys-data-on-us...