I think for a chat app Haskell may not be useful(but note that whatsapp was built on top of Erlang, a somewhat functional language)

I think Haskell and other functional languages can really improve software in domains with complex business logic and rules which is most of the software that deals with the real world and is useful to the world.

Correctness does actually matter a lot in software, nobody would trust a calculator that doesn't give the right results. Ofcourse people can tolerate edge cases but it's far better to remove them upfront.