So it is more like having one tool more in the toolbox than anything else.
Note in particular that it’s mostly developer-facing infra. The vast majority of production infra - the stuff actually running on all those thousands of servers - is still all written in C++ or Hack.
I don't know if they're planning on changing the rest of the compiler to Rust - wouldn't be surprised if they did.
Rust as a language is mature and pretty stable.
However, the ecosystem... not so much. Pretty much every crate is still at version 0.x, with breaking changes happening constantly. Major dependencies get abandoned.
What is often overlooked is that Rust projects have a very high maintenance cost. Once a project has been written, keeping all the dependencies up to date is a lot of work, that often requires complete rewrites of parts of your applications.
The dependency upgraded to Tokio 0.2.x and I whenever I spend a few hours trying to fix the compile errors I end up giving up. I like to write code to get things done and I don't take pleasure in the extra complexity that Rust brings.
Rust is present in quite a lot of spaces. But companies outside sillicon valley or even the US do not advertise what they are doing as much.
I am as far from SV as possible and a fulltime Rust dev.