1. A method performed by one or more data processing apparatus, the method comprising: receiving, by one or more first servers operated by a first entity, request data indicating that a user device has requested a content item, identifying, by the one or more first servers and based on information specified by the request data, one or more other servers to which user interactions with the content item are to be reported, wherein the one or more other servers are operated by one or more other entities that differ from the first entity; providing, by the one or more first servers and to the user device, response data in response to the request data, the response data including data that cause presentation of the content item, wherein the response data includes a hyperlink specifying: a destination web page to which the user device will be redirected in response to user interaction with the content item; and one or more unique identifiers for each of the one or more other servers to which user interactions with the content item are to be reporter; receiving, from the user device, by the one or more first servers, and after providing the response data, interaction data specifying that user interaction with the content item occurred; providing, by the one or more first servers and to the user device in response to receiving the interaction data, redirect data that cause the user device to be redirected to the destination web page; identifying, in the interaction data and by the one or more first servers, the one or more unique identifiers for each of the one or more other servers; and providing, by the one or more first servers and based on the identified one or more unique identifiers, reporting data to each of the one or more other servers, the data specifying that the user interaction with the content item occurred, wherein providing reporting data comprises providing reporting data that include an encrypted shared unique identifier.