Everyone you are friends with on FB and everyone they are friends with and so on plus everything you have done on FB such a liking pages, posts, artists, films or ads you have clicked on is used to create a detailed profile of you. If you are surfing the web while logged in to FB that activity is added to your profile. If you are using FB on your phone then your address book, calender, call logs and so on are accessible to FB.
FB will know an awful lot about you if you let them.