The idea that Microsoft doesn't use FOSS is strictly incorrect. In fact jQuery is bundled with ASP.NET MVC these days. Obviously they use Git and Mercurial source control too. This is all stuff that is publicly known, there also evidence (copyright strings, etc.) that Windows had BSD code in for awhile, and some evidence that Microsoft used Hadoop/HBase in Bing at some point (well, PowerSet was, which was bought by Microsoft - I highly doubt they threw away all of PowerSet's code on the day they acquired them).
But the thing is, it doesn't matter. You aren't a better FOSS contributor if you avoid using it. Really, the whole contention is silly.