The problem is that without the scalability infrastructure, it doesn't work. Although to be fair, the Twitter people probably didn't know that in the beginning,

The problem is that without the scalability infrastructure, it doesn't work. Although to be fair, the Twitter people probably didn't know that in the beginning, either. So you could have built a crappy prototype, true. However, while Twitter is an exciting startup, a Twitter clone is not (anymore). Granted, you might be able to build the occasional CRUD prototype for something that could become an interesting startup. I doubt it is true for all kinds of startups, though.