I think another interesting question is how hubski will financially sustain itself if it becomes really popular (whether as the "next reddit" or something completely else)? Is it donations, ads, subscriptions, dynamic membership costs (à la pinboard.in) or something else?
This is something that I have put much thought into. Among the top mechanisms is a subscription for an ad-free version of the site (with some ideas for the ad-version), or dynamic membership cost. However, there are others. I am very leery of advertising. Advertising can respect a user, but it is too common that it doesn't. Networks like The Deck do a great job of respecting users, as an example. The goal of Hubski isn't "to get big". The goal of Hubski is to provide a certain type of experience. That is something that guides my decision-making.