1. A computer-implemented method for ranking search results, comprising: under control of one or more computer systems configured with executable instructions, obtaining an initial set of search results and a query used to generate the initial set of search results, each search result corresponding to an instance of content, the query including at least one term; for each term in the query, determining a frequency with which users previously performed a selection action with respect to an instance of content in response to prior search results presented for a prior query that included the term; combining the determined frequency for each term of the query, with respect to each of a plurality of instances of content having frequency information for at least one term of the query, to generate a ranking value for each of at least a portion of the instances of content; generating an updated set of search results including at least a portion of the initial set of search results with a ranking based at least in part upon the determined ranking values; and providing the updated set of search results to a source of the query, wherein the updated set of search results is capable of including additional search results, corresponding to one or more instances of content not included in the initial set of search results, based at least in part upon a determined ranking value for each of the additional instances of content.