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.)
Hmm, if you stumble across it let me know. If you've build some secondary app that people are using you can probably get away with scraping out bits of the graph along the way, but most of the ones I've seen just do some very simple counting of friends rather than trying to do anything like finding clusters or trying to figure out which of them you're not likely to already know about.
Hey, I'm one of the guys who built WhoShouldiFollow. Glad the recommendations are working out for you.
The Twitter API rate-limiting makes things harder, but as a whitelisted app we haven't (yet) run into any situations where we've hit our quota or get throttled in any way.
(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.)