A method comprising: receiving, from a system external to a social networking system or an application external to a social networking system, a request to provide a message generated by the system external to the social networking system or the application external to the social networking system to one or more target viewing user, the request including: an identifier hash for each of the one or more target viewing users generated using a hash function from an item of personally identifiable information associated with the target viewing user and the hash function used to generate the identifier hash; accessing one or more user profiles maintained by the social networking system, a user profile including information about social networking system users; generating identifier hashes for the social networking system users by applying the hash function to personally identifiable information in the user profile about the social networking system user; for each of the received identifier hashes for the target viewing users, determining that one or more identifier hashes for the social networking system users matches the identifier hash for a target viewing user provided in the request; responsive to the determining, providing the message generated by the system external to the social networking system or the application external to the social networking system to the target viewing user via the social networking system. receiving, from a system external to a social networking system or an application external to a social networking system, a request to provide a message generated by the system external to the social networking system or the application external to the social networking system to one or more target viewing user, the request including: an identifier hash for each of the one or more target viewing users generated using a hash function from an item of personally identifiable information associated with the target viewing user and the hash function used to generate the identifier hash; accessing one or more user profiles maintained by the social networking system, a user profile including information about social networking system users; generating identifier hashes for the social networking system users by applying the hash function to personally identifiable information in the user profile about the social networking system user; for each of the received identifier hashes for the target viewing users, determining that one or more identifier hashes for the social networking system users matches the identifier hash for a target viewing user provided in the request; responsive to the determining, providing the message generated by the system external to the social networking system or the application external to the social networking system to the target viewing user via the social networking system. an identifier hash for each of the one or more target viewing users generated using a hash function from an item of personally identifiable information associated with the target viewing user and the hash function used to generate the identifier hash; accessing one or more user profiles maintained by the social networking system, a user profile including information about social networking system users; generating identifier hashes for the social networking system users by applying the hash function to personally identifiable information in the user profile about the social networking system user; for each of the received identifier hashes for the target viewing users, determining that one or more identifier hashes for the social networking system users matches the identifier hash for a target viewing user provided in the request; responsive to the determining, providing the message generated by the system external to the social networking system or the application external to the social networking system to the target viewing user via the social networking system.