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.

It's not different from a union blocking the gates of a factory so non-union workers can't get in.