... or they would start their own project similar to Unladen Swallow.
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.
Actually I think it would be different case - they would probably not need to reimplement language in their own way - because speed benefits are already there (when it comes to pypy). Instead they would patch other libs to work with pypy on a need basis. Which seems way less work than start from scratch and implement whole lang - and community would benefit a lot from this too. But since it will not ever happen it's just speculation at this point.
So having superior language with better out of the box speed in default implementation - I'd hardly call same place.