Here is my answer. App.net has positioned itself as "anti" - capturing the developers who were turned away from other social networks. That's OK as a starting

Here is my answer. App.net has positioned itself as "anti" - capturing the developers who were turned away from other social networks. That's OK as a starting point, but it doesn't account for growth to 100k users. So who are those users? And I think the answer is: they're users that would have done business directly with the outcast developers and their products, rather than App.net. App.net is a new kind of middleman. There are types of social networking services that would be suitable for subscription fees, and ordinarily each of those devs would have the option of building out their own infrastructure, or trying to cram their idea into the second-class citizenship of a Facebook or Twitter, with all the risks that entails. But hooking themselves to App.net gives them a mix of the two options - more freedom, less bootstrapping. So there's a massive positive feedback loop associated with getting developers on App.net - the same kind of feedback loop that made Microsoft in its best years so developer-centric. The downside is that everyone's going to be heavily locked in. The endgame for this ecosystem isn't likely to be pretty.