WhatsApp did modify Erlang but it's interesting that Elixir/Phoenix was able to hit the same scale on a single server without any such optimizations. [1]

[1] https://changelog.com/podcast/208

> I wasn’t thinking we could actually get WhatsApp-like scale, because when I read about WhatsApp, they were using FreeBSD and they forked Erlang and made some optimizations, they fine-tuned FreeBSD… So I was thinking that it was gonna be very difficult to try to replicate that kind of scale.

> So we were doing extra work, and it was really fulfilling to actually see that with minor changes in our initial best-effort approach with just a few tweaks was able to go to something that was able to get millions of connections. That was incredibly fulfilling to come full circle, and also it’s a great brag slide now, of showing that two million connections chart.

At that scale you are generally modifying your entire stack or building it from scratch no mater what you are using. Also they were on a pretty old version and a number of their fixes are in Erlang itself now.

WhatsApp engineers have been quite clear that using Erlang was a great choice.

Not only WhatsApp but there are no files checked into any Facebook source repository with .ex or .exs suffix and looks like Elixir.