Third-party communications to social networking system users using descriptors

Based on information used by the external system to identify a target user, the external system generates a target user hash identifier that is communicated to the social networking system along with a message. Using information from user profiles maintained by the social networking system, the social networking system generates hash identifiers for its users and compares the target user hash identifier to the generated hash identifiers hash identifiers for social networking system users are generated. The social networking system sends the message to a user associated with a hash identifier generated from user profile information that matches the target user hash identifier. Hence, an external application may communicate with social networking system users without accessing access information about the users maintained by the social networking system.