There is a functional programming language that is coupled to a platform, that most people on HackerNews wouldn't think of: the M language for PowerQuery. It is used to make ETL pipelines for Excel (and I think other Microsoft apps?). It is very popular, although among people who don't consider themselves "programmers" or "software engineers", but rather analysts.

https://docs.microsoft.com/en-us/powerquery-m/

In fact, Excel itself can be considered a functional, reactive programming environment, and if we grant that, then it is the most popular programming language on the planet.