All the comments in the thread are focussing on comparing Microsoft's contribution to Facebook or Google for one project and giving credit to Microsoft. But, it is important to remember that Google and Facebook contribute to OSS in different forms. For example, Google spends regularly on Google Summer of Code. There may be other example for Facebook as well.
I am not a fan/employee of any of these companies. I am just putting the contribution into a different perspective.
CoreCLR alone is ~2.6 million lines of code, with several million lines of code to go [1]. I would wager that by raw LOC, Microsoft will open source more code in 2014-2015 than Google and Facebook combined.
I am not a fan/employee of any of these companies. I am just putting the contribution into a different perspective.