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.