Well jayd did the same thing as that small company (which I joined in 2011 when it was small and left in 2019 when it was not so small), run ejabberd to solve a problem. In our case, Erlang subsumed pretty much the rest of our service over time. When I started, chat was Erlang, but status messages, registration, and contacts were PHP with MySQL, media was PHP (with no database), but those all got sucked into Erlang with mnesia because it was better for us.
But I guess it doesn't always work that way. FB chat was built on ejabberd and then migrated away.
Isn't there this "small" company that has a chat app that is using erlang :P