Who uses Erlang, and why?

I talk to a lot of people about Erlang, usually programmers who code in mainstream languages. A surprising number of them have heard about Erlang because of WhatsApp. It definitely helps for a niche programming language to have a famous use case.

There are many other well known examples of systems and software that use Erlang. Here we focus on 3 examples and why they are built using Erlang.