Which companies are using Erlang, and why?

At least some of those examples are highly misleading:

> There is no better example of Erlang’s reliability than the English National Health Service (NHS). [...] Using Riak (written in Erlang), the NHS has managed 99.999% availability for over five years.

From what I could find out, NHS's Spine2 is mostly Python, with "a bit" of Erlang and Javascript. They do use Riak, but they also use Redis, RabbitMQ, Python/Tornado, Ubuntu and lots of other components.

This sentence could have said "Using redis (written in C)", or "Using Tornado (written in Python)", and still be fully correct.

That Quora post refers to Facebook Chat - not WhatsApp. As far as I am aware, it is still implemented in Erlang.

That system was the backend of the functionality currently known as Facebook Messenger, not WhatsApp.

One might note, snidely [1], that three-four years after freezing development of an Erlang-based messaging system, starving maintenance work on that system of engineering effort, devoting massive engineering resources to a from-scratch C++ rewrite, and in the meantime blaming the language for relatively minor system design issues that could have been improved with a fraction of the effort (including, but not limited to, Erlang's ability to wrap allegedly critical C++ components) … Facebook plowed $19B into the acquisition of an Erlang-based messaging system.

[1] as a main author of the Facebook Chat version written in Erlang

> WhatsApp used to use Erlang, not anymore. Facebook rewrote it completely in C++.

Highly doubt it. Here is Maxim Fedorov talking about scaling their cluster to 10000 nodes in 2018 https://www.youtube.com/watch?v=FJQyv26tFZ8 it would be pretty ridiculous for them to completely rewrite that in C++ one year later.

Which is far better, IMHO