From securityboulevard.com
Facebook uses tracking plug-ins; bits of code that the consumer is unaware of, to track the behavior of users when they log out of the social media site, and which tells Facebook what websites their subscribers are visiting and what they are doing when they are not using Facebook. Using cookies, Facebook, like other sites, captures the URLs from which users are referred to Facebook, and where they go. Facebook then sells this information about its subscribers for profit.