Because they actually solve a real problem for a majority of their users? I mean I get it, I'm on the hate train too but their actual product (the Facebook web site) provides a useful service. It allows me to share a bunch of stuff with a bunch of groups of people who I want to stay in touch with, in a low friction manner.

Pretending Facebook is useless is like pretending GM doesn't make cars. Sure, maybe you don't like GM cars but they fill a genuine need for a lot of people.