An interesting thing is happening is that as we try to solve harder and harder problems, we are finding ourselves wanting to change the programming language we're living within. We have a bunch of efforts at Facebook trying to do so:
- When I joined 5 years ago, we were already adding features by "transpiling" our code.
- We are part of TC39, the JavaScript standard committee, trying to bring new concepts to the language itself.
- We are investing a lot in Flow to add an incremental type system to JavaScript
- We're investing into bytecode representation and optimizations for React Native
and finally are trying out using a completely different language that targets JavaScript: OCaml. Reason is an attempt at greatly improving the syntax, developer tooling and documentation to make it feel like a 2017 language instead of early 80s :p
One of the big issue is that we don't have a crystal clear idea of what we actually want so exploring many avenues is a really good thing to do, and I'm so glad that Facebook is willing to dedicate so much efforts into those!
I'm not super familiar with the rollout of Reason inside of Facebook, I know that Messenger.com is using it heavily and successfully and I wouldn't be surprised if there was a bunch more other things that are lower profile using it as well. You should ask this to the reason folks on discord, I'm sure they'd be able to give you a better answer.