Are you sure about that? Is Hack / PHP powerful enough to handle billions of messages an hour? If so, i'm impressed.
But I doubt it and I'm sure you're wrong about what runs Messenger. No company, least of all the Facebook scale ones use just one language for everything.
Of course the backing service is not PHP. That would be a particularly poor choice for a high volume messaging statement. They originally wrote it in Erlang and later rewrote it in C++.
But I doubt it and I'm sure you're wrong about what runs Messenger. No company, least of all the Facebook scale ones use just one language for everything.