There are plenty of large systems built on on MS technology apart from the MS ecosystem (Bing, Live, MSN, Skype) there's GoDadday, Just-Eat, Monster, PlentyOfFish, etc.
However realistically most startup founders are going to build a startup using the programming language they know or one that is particularly suited to the platform (i.e Objective-C for iOS), and few startup founders come from .Net backgrounds and even fewer choose to stick with that language.
I guess that makes me and our company unique. Started over 3 years ago on the .Net platform and have no reasons to consider switching. The BizSpark program and Azure services are great for startups.
That would be the front end; as I recall, the back end was Solaris running Oracle. (I tried to confirm my memory but the obvious keywords do not give good results with Google.)
Hotmail started out as a unix-based system. Microsoft bought it, and over a period of years moved it all to Windows. I would be stunned if there's any unix, linux, bsd, or oracle left at this point.
Yeah, we're pretty sure the back end has been converted as well.
It was amusing to watch their first effort to convert the front end fail, then when they got it right, for a while people were watching as more and more of the front end machines were switched over.
However realistically most startup founders are going to build a startup using the programming language they know or one that is particularly suited to the platform (i.e Objective-C for iOS), and few startup founders come from .Net backgrounds and even fewer choose to stick with that language.