Just charge $1/year. Seems like you could easily run reddit on $150,000,000. You could even do a free/premium model. Free users get ads and limited number of down votes and comments per day. Paid users get no ads and unlimited comments and down votes per day.
"I suspect it will be possible to create a large business with a collective identity community but it will require an unprecedented level of host-community interchange."
TL;DR You can't monetize a democracy-driven online community, but could support it with a coop model.