First, don't make a decision based on what other people are doing. Don't make a technical decision based on the job market either: that's an argument commonly used for Blub. If you have a cool and interesting project, people will not mind learning a different language to join you, even if they're mostly hobbyists who would never touch MS Software on their own.
There are some caveats, however: you should have a solid reason why you're using this product ("it's what I know how to program" or "it's easy to hire for" is not; "we have a large codebase and we can't just stop feature development and do a re-write" is), it has to the an acceptable tool for the job (e.g., you're not doing machine learning in PHP) and you have to be flexible enough to use other tools when they're right for the job (e.g., Facebook writing services in C++ when needed vs. forcing everyone to work exclusively in PHP). If you fit these criteria, you'll be able to hire hackers (although I personally wouldn't join a company that required me to run Windows on the desktop, which is very likely to be the case if your product is on .NET).
Now as for why I don't use MS tools:
F# is pretty amazing and C# is fairly bearable compared to other languages in Blub Central (of course it's still a Blub). If Mono were more usable for the server side, I'd be writing projects in those two languages. I'd very seriously consider Gtk# or Cocoa# + Mono for writing a desktop application (as opposed to using C++ + Qt or Java/Scala + Swing/SWT). Nonetheless, Mono is just not (yet) usable for a server.
I'd flatly refuse to use Windows for either a server or a development machine as it's simply an environment designed for the masses and not for hackers. I am many times more productive with Perl/Python UNIX shell + Terminal than I am with a GUI and Powershell (which is a lot better than the Microsoft tools I grew up with).
If Microsoft actually targeted hackers and developers as opposed to IT departments of non-technical firms I'd consider their products. Problem is that they're not a charity, but a corporation with a legal obligation to generate profit for their shareholders. Their current strategy works well for them and I don't see that changing. When I see a company using a proprietary stack, I first assume that the company is mostly "business guys" and wants warm bodies. I'll have to convinced otherwise, which is possibly; I am not close minded: you can't just judge people by the architectural decisions they made without knowing why the made them.
I am glad to see Microsoft support F# and Scala on .NET, but I am not sure how far this will go. I'll stick with an open source/Linux stack for all my hobbyist programming and I can't imagine building a start-up on anything else.