15. A data storage device storing instructions that, when executed by one or more computers, cause the one or more computers to perform operations comprising: identifying a content source that was referenced by search results for each of two or more different queries that have been previously received from users; determining, for each query from the two or more queries, a number of user interactions with the search results that referenced the content source and were provided to the users in response to the query; ranking the queries based on the number of user interactions with the content source when presented to the users as a search result for each query; determining that a term provided by an advertiser matches a query from the two or more queries; determining that number of user interactions with the content source corresponding to the matched query meets a threshold number of user interactions; and providing data to the advertiser identifying the content source as a presentation location for the advertiser's content.
15. A data storage device storing instructions that, when executed by one or more computers, cause the one or more computers to perform operations comprising: identifying a content source that was referenced by search results for each of two or more different queries that have been previously received from users; determining, for each query from the two or more queries, a number of user interactions with the search results that referenced the content source and were provided to the users in response to the query; ranking the queries based on the number of user interactions with the content source when presented to the users as a search result for each query; determining that a term provided by an advertiser matches a query from the two or more queries; determining that number of user interactions with the content source corresponding to the matched query meets a threshold number of user interactions; and providing data to the advertiser identifying the content source as a presentation location for the advertiser's content. 20. The data storage device of claim 15 , wherein the one or more computers perform comprising: receiving, from the advertiser, a bid corresponding to placement of an advertisement on the content source; and including, in an advertising campaign for the advertiser, the content source as a presentation location for one or more advertisements included in the advertising campaign.