Windows + Ubuntu + CentOS ... I'm curious to know how the OS selection was made and specifically why the HAProxy servers are Ubuntu and the Redis servers are CentOS. Do your admins work on all three OSs?

It's pretty simple, we started off on Ubuntu for our linux boxes. Then made a decision to move to CentOS for the stability and vendor package compatibility for our core services. However, We are going to stick with Ubuntu on some of our back end servers - management and utility servers for the larger package repositories. Our Core Q&A engine is written in C# with MS-SQL databases so obviously we need to run windows for that.

There are only 2 sysadmins so yes we work on all OS's

Any good Linux admin should be able to work on any distribution... and will... as long as you allow them to complain about it.

Other than microsoft themselves: myspace is still fairly huge but probably a mixed stack. Plentyoffish is up there too - http://highscalability.com/plentyoffish-architecture

I have also ran .NET backed stuff that was larger than this... It's not that uncommon. In my experience the guys running .NET backed stuff just aren't usually as vocal about things. ;) hi5 (the social network) also runs a lot of .NET and SQL Server nowadays. They're also running Windows Server: http://www.microsoft.com/casestudies/Case_Study_Detail.aspx?...

A (large) previous company of mine used .NET for an internal operation which was a high volume database oriented application. Assuming it's still in use, I'm reasonably certain it did quite a lot more volume than that (orders of magnitude more), but it wasn't a website.

We do many more hits a day with IIS and .Net on windows. And we're far from the biggest.

Mind if I ask who 'we' is?