Well, to make this clear, Microsoft is just one company that has purchased ads on Polygon. Other ad buys include PlayStation, Speed Stick, Head & Shoulders, Qualcomm, Ford and others that I can't remember. I think some publishers like Deep Silver and maybe Capcom also have purchased ads? None of those companies "funded" Polygon in the sense that it's accused of.
To the best of my knowledge, Microsoft's ad buy was related to videos. Whatever money they spent in advertising was mostly shown in front of and after video features like the Human Angle series and Press Reset. It's an advertising deal like thousands of other advertising deals related to websites.
The real
funding of Polygon comes from its parent company, Vox Media, which also owns The Verge and SB Nation. Vox Media itself was funded through various investments. These are, according to
Wikipedia: