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.