Not legally they haven't been. Facebook discovered this recently, when they attempted to lay off people in NL. They discovered that because it's a mass layoff, they need to consult a workers council. But they don't have a workers council, so they need to elect one, which will take three months. Then another three months of consultation.

Even in Ireland/UK which have pretty employer friendly labour laws, none of the FB employees have actually been laid off yet, they're still in a consultation period (till December I believe).

Unfortunately for US CEO's, the US labour law does not apply globally.

Strong unions could do industrial action to take down the site. Take the server offline.