Scaling: 0 to 20 million users

A story of how our infrastructure evolved over time to accommodate an increasing number of users - from on-premise to cloud and back down.

How does one make an infrastructure to handle more than a couple of users? 

How do you go from 100 to 1000 to 100,000 to tens of millions? 

What happens when due to popular demand hundreds of thousands of users hit your servers at the same time?

I'll tell you a story of how a small team of people managed to move software and services from one server to two, and then to dozens on cloud and then back to on-premise.  What we encountered on the way, where we failed, and how we solved it.