As far as I know, Facebook has possibly the biggest Haskell deployment in form of one of their anti abuse tool.
I think it is a good tech choice to be honest, I want my blockchain carefully built and audited.
https://engineering.fb.com/2015/06/26/security/fighting-spam...
I’ve seen a small team of functional programmers tackle projects that would require dozens of OOP programmers. The challenge is finding them!
I worked with a guy who was an serious Lisp-hacker who could do fantastic things in Lisp and was always advocating that we do certain things in Lisp. But he was also a great programmer when 'forced' to use C++ or Perl, he just grumbled more while programming.