At the far end of this reasoning three separate companies that all use AWS share the same underlying infrastructure and all go down when AWS does. It doesn't mean they're not independently managed.
And the same could be said for the electrical grid, the post services, roads, etc. I agree that infrastructure and monopoly are two very distict concepts.
So, I get you here and what you're saying. But companies at this scale argue they're "independently managed" or whatever.
With that context, for Facebook, there's really no way of saying that these 3 different brands are independent when it comes down to it.
It'd be much like Procter & Gamble saying their 100s of brands were all really different institutional entities.