From NY Post:
Facebook knows what you like. It knows what you don’t like. It probably knows whether you have been naughty or nice, and will be selling that data to Santa this Christmas season.
A standard rule of economics. If you are getting something for free then you are the product that is being sold.
I think Facebook provides a service, and so do over a billion other people. However Facebook could not exist if it did not have something of value to sell. That something is you. In theory it is possible to stop Facebook from tracking you. You should sign out every time you leave Facebook and set your privacy settings higher than the basic default. It is possible but very few garden variety users ever do it.