All of the “www” code (backend which serves all the web apps and APIs) is Hack. And a lot of that code is generated via internal frameworks like Ent which is a graph abstraction over database access.

There doesn’t seem to be any incentive to switch www away from Hack: it wouldn’t reduce the learning curve very much due to all those internal frameworks, and would remove the opportunity to optimize the language for the www codebase.

There are open source languages used in the internal backends though (services behind www). Most of it is C++.

For anyone interested, it looks like there's an open-source version (close, at least) to Ent at https://github.com/ent/ent.