Facebook has 1.393 billion active monthly members from across the world. all of those members are giving away information about themselves to Facebook, a company that saves personal information and then uses it for many purposes. Facebook data is also used by used by security agencies including the NSA and GCHQ.
Facebook knows a lot about you: name; email address; gender; date of birth; family members; friends; education; sexual preference; relationship status; religious views; your job; your location; political standing; address of your own website or blog; favorite music, books, TV shows and films; phone number; messages you post on your own timeline; messages other people post on your timeline; contents of private messages; sites you frequent; topics you talk about; and what you look like.
You can read more in an article by Philip Bates in the MakeUseOf web site at http://www.makeuseof.com/tag/facebook-privacy-25-things-social-network-knows.