Agreed. My experience is that optimizing and scaling your app from day one in preparation for Twitter-like traffic is a self-defeating prophecy. Web app scaling is hard, and if you spend so much effort on it from the beginning it will leave little time for actual development, and as a result your web app will most likely fail to become popular.

I'm convinced that scaling is something you need to worry about after you become popular. There's no point to worry about it any earlier.