Facebook was build from PHP, but they later created their a PHP transpiler HipHop for PHP which was later replaced by a virtual machine HHVM. This VM runs Hack which is suppose to be a dialect of PHP.

Discord too is using Elixir. They also combine it with Rust in some places where performance really matters.

Some big names I’ve seen/spoke to/been aware of:

Guardian: Scala
The Financial Times: NodeJS
Daily Mail: Clojure
Walmart: NodeJS
Heroku: Ruby, Go, Erlang
Atlassian: Java
Tesco: C#, NodeJS
Reddit: Python
Google: Java, C++, Python, Go
Nationwide Building Society: C#, NodeJS
Santander: A proprietary graphical programming language, C#
HSCB: C#
Wikipedia: PHP

I was talking to a Pinterest employee last year and they said that they stopped using Elixir.