jet.com https://tech.jet.com/blog/2016/02-05-jet-engine-built-2015/ . F# on Azure. Acquired by Walmart for 3 billion in cash.
From the article "in this case for massive distributed systems Functional Languages are the right tool ...
Immutability at scale is really important because it allows you to go from 30k members to a staggering 2.5 Million+ and not even feel blip within 3 months, like we did here at Jet from the end of July to October. That’s powerful!"