Nothing wrong with .NET - perhaps one of the reasons it doesn't look like it is being used is because it's more corporate. Startups don't use M$ for a variety of reasons....cost is a big one - have you seen licence fees for sql server? insane I tell you! That said, I believe stackoverflow is .NET MVC so...not huge like facebook, but certainly an excellent product with an active community.

.NET, like any framework, has its quirks.....it can be a little frustrating sometimes with the way it tries to abstract you from the actual GETs and POSTs in webforms, but there you go. I do think the IDE and debugging is something that works fairly well in .NET. In fact, I often wonder if Microsoft couldn't just market their IDE and make it compatible with PHP/Ruby/Python etc rather than ASP per se.

At the end of the day, I like to think programmers use the tool best suited to the job at hand - languages are the tools, learn .NET, learn php, learn Ruby, heck dabble in everything that looks interesting even if just to get a feel for it - then you can make informed decisions based on experience.

Same with OS - doesn't matter which platform you are best on Windows/Linux/Mac - but at least get a feel for the others.