Hey! It's weird: it's not the first time we get questions like yours and I'm not sure where such confusion comes from! Reason and React are two things. Reason's a syntax on top of OCaml, the programming language. ReactJS' a UI library. Messenger.com _is_ written in React, lots of which are, specifically, in ReasonReact, which is Reason bindings for ReactJS (the rest, plain ReactJS).

Reason's open-source, as well as ReasonReact(https://github.com/reasonml/reason-react). Using BuckleScript the OCaml-to-JS compiler, we compile to normal JS code with minimal runtime.

Here's why we made it: https://reasonml.github.io/guide/what-and-why