10. A computer system for personalizing placed content associated with a search query, comprising: a user profile, based, at least in part, on query terms in a plurality of previously submitted search queries; and a placed content server, including a plurality of placed content, for identifying a subset of the plurality of placed content that matches a search query and that assigns a popularity score to each of the plurality of placed content in the subset in accordance with the user profile, a respective bid value for the placed content, and a respective click through rate for the placed content based on multiple user activities, and that ranks the subset based on the respective popularity scores of the subset of placed content, and that prepares for display of the subset of the plurality of placed content in an order determined by their respective popularity scores, wherein the popularity score of each piece of placed content is based on a similarity score between the user profile and a placed content profile associated with the placed content, wherein the similarity score is indicative of a level of similarity between the user profile and the associated placed content profile, and wherein the similarity score is combined with the respective click through rate and the respective bid value to determine the popularity score assigned to the placed content.
13. The system of claim 10 , further including a scaling factor associated with the similarity score.