For doing software-defined networking, Microsoft is using Linux and they seem to have good things to say. "The Azure Cloud Switch (ACS) is our foray into building our own software for running network devices like switches. It is a cross-platform modular operating system for data center networking built on Linux. ACS allows us to debug, fix, and test software bugs much faster. It also allows us the flexibility to scale down the software and develop features that are required for our datacenter and our networking needs."
If you want to learn more about Microsoft's Linux-based Azure Cloud Switch, read this Microsoft Azure blog post.