I'm not sure this outlines the 4 dominant models. It strikes me more as the dominant business model (per-click ads) and 3 ideas that are novel and popular. The

I'm not sure this outlines the 4 dominant models. It strikes me more as the dominant business model (per-click ads) and 3 ideas that are novel and popular. The latter 3 haven't proven themselves as "Business Models". There is no particular indication that they won't go the way of (e.g.) Google's custom search: a good idea at the time, but unfortunately not particulary relevant to the market in the long term.