I’m not sure about privacy settings on twitter. But on fb, you can’t send me a friend request unless you are friends with one of my friends. And there is a few other things that factor into it.
But it basically boils down to, fb is easier to monitor…and the dark side of fb is harder to find. (Plus, I’m not even interested in twitter, instagram, tumblr, snapchat, etc) (fb msgr is trying to become snap chat)