Zuck is just bullshitting here, like most of what he says.
There is zero chance he wants to pay even a single person to sit and take calls from users.
He would eliminate every employee at Facebook it it were technically possible to automate what they do.
He can definitely fire most people at Facebook. He just doesn't because it would be like not providing a simple defense against a pawn move on a Chess board. No point in not matching the opposition's move if you can afford it. They hire, we hire, they fire, we fire.
So would everyone that ever created a business. Nobody grows headcount if they don't have to. Why be responsible for other people's livelihoods if you can make it work with less people? Just more worries and responsibilities.