Here's the customer showcase on the .NET site (with links to more at the bottom of the page). It lists some of the biggest enterprise adopters: https://dotnet.microsoft.com/platform/customers

Of course, lots of Microsoft services run on .NET Core. Last year, the Bing team talked about how their move to .NET Core 2.1 gave them big performance jumps here: https://devblogs.microsoft.com/dotnet/bing-com-runs-on-net-c...

(disclaimer: Microsoft employee, .NET team)