Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for enhancing online user privacy. Methods can include receiving tag information specifying a given publisher identifier for a publisher and a given client identifier assigned to a user of the client device by the publisher. A given service identifier assigned to the user by the service apparatus is obtained. A mapping between the given service identifier to the given client identifier is created. A list of client identifiers assigned to a set of users by the publisher is received. A list of matched service identifiers corresponding to the list of client identifiers are stored. Multiple content requests are received from multiple different client devices accessing services provided by the service apparatus. Responses to the content requests are based on whether the client devices provide service identifiers that are included in the list of matched service identifiers.