From an infrastructure PoV, I seem to recall that WhatsApp was one of the few major companies that used Erlang, and were famous for being able to run the entirety of WhatsApp on only a few servers, each of which was serving millions of concurrent connections, mostly thanks to Erlang/BEAM (at least, from what I read). When it got acquired by Facebook, they then proceeded to rewrite the entirety of the backend in C++. Seems kind of baffling to me.