If they started with Python (or anything else) they would have ended up in the same place. They would end up writing their own compiler and VM because they want and they can. It's actually amazing that they didn't extend the language itself yet.
So having superior language with better out of the box speed in default implementation - I'd hardly call same place.
I think picking simple language like PHP makes it much easier to create your own fully custom stack once you need one.
Ofcourse Facebook problems will never be your problems so every one should pick what is right for him at any given moment and stop dreming about (im)possible future.
I'd say that if Facebook was java based they'd have to stick with hotspot vm. Whether it's a good thing is a completely different story.