A point that bears repeating, by Bryan Doll:
I’m not suggesting anyone stop building prototypes. I’m not suggesting we don’t explore the idea of an application through its development. However, the next time you have that “aha” moment, think first of the value you can create and the app will follow. The app, it turns out, really is the easy part.
A startup is not a web application. A startup is first and foremost a business, and to be a successful business it needs to fulfill some important function for its customers, so that they will be willing to pay for it.
Of course, this doesn't mean that every startup needs to follow this approach, but Twitters and Facebooks are few and far between.