In short: to keep doing whatever they want without you opening your mouth.
To elaborate: They don’t all pay well. Not even close. And none of them pay workers in other countries well, and they can outsource endlessly with no repercussions.
But they want to keep their home in the US (the world military empire) because it lets them do whatever they want. And they don’t pay migrants nearly the same because migrants are bound by their visas. But too much of that and the Americans will catch on.
Unionizing in this case is extremely unlikely. There is too much for the employees to lose. Instead, Facebook and the US Government will continue doing whatever they want.