They use(d) Elixir (based on Erlang VM) [1]. WhatsApp apparently considered Erlang to be one of the reasons why they could serve such a massive userbase with a tiny team [2], which I guess is a tech advantage.
In any case, I enjoyed reading Discord's tech blogs on Elixir.
[1] https://blog.discord.com/tagged/erlang
[2] https://www.wired.com/2015/09/whatsapp-serves-900-million-us...
EDIT: added reference and clarified that I don't know if they still use Elixir.