To be honest I don't think there's a business model here.

If you host just a few communities - there's not much difference from just hosting separate websites.
If you host a moderate number of communities - the cost maintaining the infrastructure, branding, etc, will not be met by whatever each of the communities combined can pay.
If you host a very large number of communities - things have to be incredibly optimised so there's no support requirement by each site. Which isn't really possible for a complex product like Composr, only for something simple like Facebook Pages (which would be an insurmountable competitor in that case).

So, while it would be great if people set up more community hubs with Composr, promoting that would not make it's money back (because those sites couldn't pay us much for the development), and is pretty niche.
So our promotional strategies need to be much more scalable, or be directly profitable.