Methods, systems, and apparatus, including computer programs encoded on a computer-readable storage medium, and including a method for conversion tracking. The method comprises anonymously linking plural identifiers, each identifier associated with a user and specific requesting source. Anonymously linking includes linking different identifiers associated with different requesting sources used by the user to access content. Anonymously linking includes linking the different identifiers using an identifier that does not include any personally identifiable information, and storing impression, click and interaction data for the user in association with respective identifiers. The method further comprises determining that an impression of content to the user has occurred on one of the two different requesting sources and that an interaction, associated with the impression, has been recorded on another of the two different requesting sources, and linking the impression and/or click with the interaction, including charging a sponsor associated with the impression and/or click.