A social networking system may provide user metrics for unknown dimensions to an external system using received hash codes from user devices performing actions on the external system. Embodiments of the invention receive hash codes generated by external systems that are embedded in redirect Uniform Resource Locators (URLs) that are executed by user devices. The social networking system may match the received hash codes to identifying information about users, e.g., user identifiers. The user profile information may be reported to external systems based on hash codes in an aggregated format without disclosing identifying information about users. In one embodiment, a hash code may include identifying information of an administrator of an external system to enable the administrator to merge information about users of the social networking system with other unknown dimensions of information about the users that is known by the external system.