In recent months App.net has started paying developers. I don't think developers ever had to pay to use the platform, although each developer probably has a paid personal account (in other news, iPhone developers buy iPhones).
I am a little concerned that a centralized silo with a sustainable business model is still a centralized silo and I personally would prefer a personal cloud/indieweb approach instead, but App.net is hardly some kind of Ponzi scheme.
Why don't developers and users get a share of the business? Didn't the developers take on the most risk a year ago? Seriously, it's about time backers of crowd-sourced projects start demanding something tangible instead of just forking over cash to every salesman to come knocking.
Anyway, eventually the investors - and the rest of the bandwagon which is sure to follow - are going to want a return on their investment, so we know how this API-feel-good story ends.
Any developer building on top of App.net expecting long-term stability is kidding themselves.