Not sure about SML but its close cousin OCaml (which I learned after SML) is used a lot in industry by some big names:

* Facebook - OCaml for Hack, Reason, Flow - static analysis and compilers

* Jane Street - heavy user for their entire backend

* Bloomberg

* Citrix

etc https://ocaml.org/learn/companies.html

Haskell is also closely related but different as it has lazy evaluation unlike SML

https://wiki.haskell.org/Haskell_in_industry

I was taught SML as a first year CS student to learn the basics of programming (although I had already used C-like languages). ML stands for Meta Language - it was originally designed and used to prove theorems. That's why SML itself tends to be quite an academic thing to this day.