I make good use out of facebook, I'm not paying for it, and by letting them know what other websites I go on, I can get more adverts for products that I might actually be interested in.
I can understand why some people would be upset by tracking cookies from unsavoury websites that want to sell your personal information for no benefit to you, but for websites that I use (and am exposed to adverts on) all day, every day, like Facebook, Google or CFC, I actually want them to know as much about me as possible, so that they can show me all the cool things I can buy, places I can go to, events I can attend and cool stuff that I can experience.
EDIT: That being said, I should say that I like that this product exists. We should always have the option to opt out if we want.