I would guess at least number of backend services for Xbox One titles. I remember seeing an advert for Titanfall being run on Azure. That was a few years ago so my memory may be flawed.

I suspect 343 Industries (Microsoft Studio behind the latest Halo games) also uses it. They are one of the big users of Project Orleans https://dotnet.github.io/orleans/, a C# distributed actor framework. Most of their documentation and samples are shown using the framework in Azure.

Visual Studio Team Services.