Everyone else is responding with FOSS, so I'll respond with some companies:
Co-Star, the astrology SaaS, is apparently written with a Haskell backend. I'd love to have seen the casting call for that.
I believe the Mercury bank also runs most of their backend stuff on Haskell. Functional languages in general are surprisingly common among financial investment firms.
Some of Target's stuff is written in Haskell. I think there was at least one big Facebook project that was written in Haskell, but they may have moved away from it by now. Awake Security does some Haskell stuff.
One thing which might be surprising is Haskell is apparently quite strong for general backend web dev.
Co-Star, the astrology SaaS, is apparently written with a Haskell backend. I'd love to have seen the casting call for that.
I believe the Mercury bank also runs most of their backend stuff on Haskell. Functional languages in general are surprisingly common among financial investment firms.
Some of Target's stuff is written in Haskell. I think there was at least one big Facebook project that was written in Haskell, but they may have moved away from it by now. Awake Security does some Haskell stuff.
One thing which might be surprising is Haskell is apparently quite strong for general backend web dev.