A method for harvesting application metadata includes receiving metadata from a web application and storing the metadata to a metadata collection. The metadata indicates a characteristic of a web application user. The metadata collection identifies the user and may include metadata from other web applications. The advertisement is then characterized. For example, keywords may be associated with the advertisement. An affinity between the advertisement and the user is then determined and an indication of the affinity is recorded. When the affinity exceeds a predetermined threshold, the user may be identified as a potential target of the advertisement. The service provider may then identify the potential targets to the advertiser or provide the advertisement to the targets on behalf of the advertiser. The web application may be compensated for providing the metadata. The compensation may occur in response to the metadata resulting in identifying the user as a potential target.