In TV, the advertisers are the customers and the people at home watching are the audience. What is being sold to the customers is the attention of the audience.
You are the product.
It costs to have a product to sell and, in Facebook's world, as in Google's they pay their product in the services they offer and it costs them in servers and running costs and salaries to make and support it all.
Just like when selling a chocolate bar at a corner store, the idea is to sell it for more than it costs you to obtain the product, stock, store and sell it.
Just accept that this is the way it works - your attention and your information are the products and they buy them from you buy providing you with services to use and then sell those products to their customers, the advertisers.