He doesn't have a role at Facebook anymore AFAIK, although I can't find the source now. I remember hearing it was mostly due to filtering engine at Facebook being rewritten in Hack, so they don't have the need for Haskell people anymore.

He posted about him leaving Facebook on Linkedin: https://www.linkedin.com/feed/update/urn:li:activity:7302785...

He also reflected on his time and Haskell usage at Facebook here: https://www.youtube.com/watch?v=gEWBHP0PvRw&list=PLQpeDZt0_x...