Why do the codebases need to share a similar set of languages and tools? Integrating two services doesn't necessarily mean merging them into a single product; Facebook could likely allow the two to communicate without a complete product merger.

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++.

https://www.quora.com/When-did-Facebook-switch-away-from-usi...

statement -> service

sigh