I heard this a couple of weeks ago (on Hacker News), and I was SHOCKED!
I have worked for years as a .Net consultant, and I had no idea Azure had grown this quickly. A couple of years ago, I thought their entire set of literature was counter-intuitive, and I quit paying attention to them.
How on Earth did they grow to be in AWS's league so quickly? Surely just getting the "default Microsoft business" wasn't enough to put them at $1 billion ??