19. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: obtaining information associated with a mobile application of interest; determining that the mobile application of interest is not from a first mobile application platform; determining that a counterpart application to the mobile application of interest exists on a second mobile application platform; generating a respective similarity score to the application of interest for each of the counterpart application and a plurality of similar applications to the counterpart application by querying a database of structured data; determining a plurality of keywords from the counterpart application and the plurality of similar applications to the counterpart application having a threshold level of statistical information relating to a respective performance of each of the plurality of keywords; expanding keywords of the mobile application of interest based on statistical information associated with the counterpart application and a plurality of similar applications to the counterpart application to use for identifying content items for presentation on the mobile application of interest by: calculating a projected statistical value for a new keyword using a weighted least regression of the statistical information relating to the performance of the plurality of keywords weighted with the respective similarity score of an associated similar application, and determining the projected statistical value for the new keyword exceeds a threshold value.
20. The system of claim 19 , wherein determining that a counterpart application to the mobile application of interest exists includes using one or more of an application index and a developer index generated for one or more mobile application platforms.