9. The system of claim 8 , wherein providing data to the advertiser comprises providing a list of content sources ranked in order of the number of user interactions relative to the matched query.
8. A system, comprising: a data store storing, for each of two or more different queries that were previously received from users, data identifying content sources that were identified by search results provided in response to the query and data specifying whether users interacted with the search results; and one or more computers that interact with the data store and perform operations comprising: identifying a content source that was referenced by search results for each of two or more different queries; 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 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 the 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.