Facebook chat did use erlang (Ejabberd to be precise), but they migrated to something other. WhatsApp uses erlang and is even a member (logo in partners section). So, Facebook uses erlang, but not directly as Facebook, but they changed tech stack away from erlang.

Facebook Messenger is built with ReasonML

The frontend is built with ReasonML on the web, and some other stuff on mobile. What the backend is built with is separate.