1. A computer-implemented method comprising: identifying, by one or more computing devices, 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, by one or more computing devices and 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, by one or more computing devices, 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, by one or more computing devices, that a term provided by an advertiser matches a query from the two or more queries; determining, by one or more computing devices, 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.
7. The method of claim 1 , comprising: determining that the number of user interactions with multiple different content sources meets the threshold popularity relative to the matched query; and suggesting different sets of the multiple different content sources in response to different requests for content source suggestions.