Facebook is a social network, and being social means sharing one’s personal information with others. In Facebook’s case, you share your information by default—both with your friends and with Facebook and its partners and advertisers.
Unfortunately, all this sharing poses a problem if you’d rather keep some things private. If you share everything with everyone, then all sorts of information can get out—and be seen by people you don’t want seeing it. Keeping personal information personal on Facebook is possible, but it requires some work on your part.