There's definitely a cost-benefit scenario, but I think in Twitter's case much of their run-away usage has been because exactly that ecosystem exists. It's like an early adopter's wet dream. My biggest gripe with it is the rate limiting since it means I can't actually build the apps that I want to.

(I'd love to build a friend-finder for it. Theirs sucks. That's a perfect setup for our graph discovery stuff, but we need to be able to traverse several levels out to do something useful with it.)