Earlier quoted context omitted.
Probably depends on the definition of leading; I'd guess that Haskell is probably used more than any other (Turing-complete, to exclude Gallina, etc.) functional language within programming language research, but you're right that in terms of usage in industry etc. Scala is probably on top.
Programming language research is a pretty small and arbitrary space with regard to overall usage, isn't it?
If leading refers to overall usage in industry, then sure, it's not really relevant.