1. A computer-implemented method of personalizing placed content associated with a search query, comprising: at a server system having one or more processors and memory storing programs executed by the one or more processors: receiving a search query from a user; accessing a user profile associated with the user, wherein the user profile is based, at least in part, on query terms in a plurality of previously submitted search queries; identifying a set of placed content that matches the search query; assigning a popularity score to each of the set of placed content 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, wherein the assigning the popularity score includes: determining 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 combining the similarity score with the respective click through rate and the respective bid value to determine the popularity score assigned to the placed content; and ranking the set of placed content according to their respective popularity scores; and preparing for display to the user at least a subset of the set of placed content in an order determined by their respective popularity scores.