I would, however, encourage you to use .NET MVC instead of WebForms. The former is very similar to Rails, Django, and other popular MVC frameworks, so if you find that you'd like to make a transition to another language/framework, it will be much easier this way. The latter is really just an antiquated technology with lots of problems and not very much of an OSS community.
At the end of the day, it's about getting to market fast and testing your hypothesis. Building your MVP is just the very beginning of your startup journey. You will probably have to change your product a lot, so go with what you can hack in quickly. If it turns out that you have the next StackExchange (which is built on .NET MVC) and you need to massively scale, then drop Spolsky or Atwood a line and I'm sure they'll be happy to give you some pointers. That's a good problem to have.
[2] http://www.nashcoding.com/2011/04/10/building-a-startup-part...