Mark Russinovich, Microsoft Azure's CTIO explained: "SONiC is a collection of software networking components required to build network devices like switches." Together with SAI (Switch Abstraction Interface) SONiC gives cloud operators "the final piece of the puzzle in delivering a fully open sourced switch platform that can share the same software stack across hardware from multiple switch vendors."
True SONiC requires Linux, but as Microsoft states right upfront in the SONiC GitHub FAQ:
Q. Is SONiC a Linux distribution?
A. No, SONiC is a collection of networking software components required to have a fully functional L3 device that can be agnostic of any particular Linux distribution. Today SONiC runs on Debian.
If you want to use SONiC, albeit without any direct Microsoft support, you can. According to the FAQ, "SONiC requires Linux kernel 3.16. We built and tested on Debian Linux, but theoretically any distribution could be supported. It is fully open sourced [under Apache and GPLv2 licenses) and can be customized by users."
Further, Microsoft will release SONiC binaries. To be more precise, "We will build some binaries, use binaries from existing Linux distributions and in some cases just release source code. The getting started guide covers how to build, install and configure SONiC."
Support? You'll need to look to the community. That said, "Microsoft is interested in keeping SONiC relevant, reliable and stable. We run it in our own network."
This is all well and good, but at day's end Microsoft is offering an open-source networking switch stack on top of Linux. That's noteworthy, but it's no "MS-Linux."
I expect, based on both Microsoft and Microsoft partner sources, to see Microsoft Dynamics ERP, Exchange Server, and SharePoint Server be made available on Linux on the Azure cloud later this year. Microsoft will also continue to build out client applications on Android. I see no reason to think that Microsoft will put any real energy into applications, such as Skype for Linux, on the Linux desktop. At most Linux desktop users might get a point release of Skype.
Then, towards the end of the year, it's just possible that Microsoft will introduce its own cloud-based Linux server distribution.