Earlier quoted context omitted.
Downloading .exe files to Linux computers seems to be a very Microsofty thing to do.
I SERIOUSLY doubt MS was giving them architectural guidance on how to make this work. Probably would have more to do with the architectural experience of the devs.
Remember that this video chat is not a web page. It's a Microsoft (and Apple? I don't know) desktop application that integrates into their web page.
I'm suggesting that Facebook is showing their alliance to Microsoft by ignoring some other OSs and launching Microsoft-powered features. If you include video chat as an integral part of Facebook, then you can no longer call Facebook a web site.