An online system receives a variety of identifiers associated with a user of the online system and generates a federated list of identifiers for the user that includes each of the received identifiers. Identifiers may be browser identifiers, device identifiers, Internet protocol address, personally identifiable information, or a user identifier of a different online system. For each identifier in the federated list of identifiers, the online system generates metadata information such as a confidence score indicating a degree of certainty that the identifier can be used to accurately identify the user of the online system. The online system aggregates features associated with the identifiers in the federated list of identifiers to generate a comprehensive user profile of the user and uses the comprehensive user profile to better serve the user.