The linked article provides a deep analysis of the stackoverflow architecture.

They use 11 IIS servers, 4 SQL Servers, Redis, Elasticsearch and HA Proxy in front of the web heads.

http://nickcraver.com/blog/2016/02/17/stack-overflow-the-architecture-2016-edition/

It is a very interesting article. They handle a massive number of hits every day, 209,420,973 hits. with a bunch of servers

It is rather a surprise the fact they are not using any cloud solution, would be interesting to know why.