Neat idea. How do you plan to monetize?
Seriously, do you have any plans of how to make money? The server costs on this could get really high if it actually becomes popular. Then you have a hugely successful app that is actively losing you money.
I'm just a bit annoyed at all the, "We'll get a few million users and then worry about how to make money." When I see a service I like that doesn't have any way of making money I'm scared it will go away. Note this isn't how the masses think, but it is how software devs think.
I also agree with you that all startups should have a clear path to monetization. I would assume that institutional investors would require as much before investing, but I think that would probably be assuming too much these days.
However, I don't think that all startups should "turn on" the monetization immediately. For those us trying to build two-sided platforms, we need to get one side of the platform on first. Typically this will be the users as brands and retailers are still very much focused on reach and are very slow to make decisions.
Some of the other things you mentioned I don't think would hinder growth at all. i.e. maybe in app purchase for the private section or sponsored challenges etc. It just seems to me too many good services get shut down because people don't think about how it is going to be profitable.