The same way all media makes it's money, selling access to their client base to advertisers.You know how social media is generating revenue ?
I'm beginning to feel a bit sorry for Facebook at this point, it's not like they invented racism or con men. All they really did was create a platform. People abused it. Maybe Facebook could have been more proactive but when these companies were set up they were supposed to be free and open to everyone. They didn't realise most people are asshats and need to be coddled like children.