Methods, systems, and apparatus include computer programs encoded on a computer-readable storage medium, including a method for providing content. A linking is provided of identifiers for users or devices known to a content delivery service. Each identifier is associated with the user or device in a specific context. The linking includes first and second identifiers. A request for content is received that includes either the first or second identifier. Bidders in an exchange are identified that have expressed an interest in bidding on a content delivery opportunity associated with the user or the device. For each bidder, a relative value is determined for a respective bidder for the opportunity to present content to the user or device in association with each identifier known to the bidder. For each bidder, a real-time bid request is generated in the exchange that includes the highest value identifier for submission to a respective bidder.