1. A method for utilizing a user of a search engine's propensity-to-click on advertisements, the method comprising: measuring two or more metrics relating to the user's propensity to click on advertising; calculating a propensity-to-click score for the user based on the two or more measured metrics, wherein the propensity-to-click score for the user indicates the user's propensity to click on advertising; and dynamically determining, using the propensity-to-click score for the user, how to display advertising to the user on a results web page presented in response to a search by the user, wherein the dynamically determining includes determining to display zero advertisements on the results web page presented in response to the search when the user has a low propensity-to-click score.