Great questions.

Re: users, you can define "users" however you'd like, if you do so clearly on your own site where people can download your software and code. We're working on making the definition of "user" clearer; that's part of why Fair Source is only v0.9 right now. In some cases you'd want "users" to include end uesrs, and in some cases you wouldn't, so we need to figure out how to accommodate both cases.

Re: why "number of users" not Unreal Engine's model (paying $20/month to see source code) or the id Software/Quake model (open-source the previous version): Those work for libraries, but Fair Source is intended for end-user applications right now.