You're forgetting that by the time app.net came up twitter was already "the" platform for this sort of stuff. App.net didn't improve on the model in any way. No awesome features, no additional value. It tried to compete with a service that's already used by millions with terrible branding (starting with the name) and a pricing model that's good for SaaS companies (because they work with business that use the solution to make money) with consumers (who don't).
Having a business model is nice. Having the right business model is essential. Twitter has a business model now, and while it isn't ideal it's in line with their type of product/company and if you scroll through some of the other comments this model isn't all that bad.
Twitter can improve it, but that's a story for a different thread.