How do Facebook ads know EVERYTHING?

[Shriekingqueen296]

Sounds kind of stupid, but I legitimately want to know! My roommate and I were trying to figure this out when we went on Facebook and we realized that there was an ad for the brand of birth control she uses and it had a little ad about a sustainability class offered at our college that she's thinking about taking...neither of which she has mentioned on facebook anywhere. Then, I went on mine it had an ad to download the song I was listening to on iTunes. It's fucking weird and I want to know how it knows everything!
Thanks you guys! We were like freaking out haha we must have called like half the girls in our sorority trying to figure this out (of course they didn't know, but we were bored so what the hell). Definitely pretty creepy though! I'm not really down with all my stuff just being looked through by some random computer thing, but I guess that's what you get for using the internet!