An online system matches a user with a user of a third party system by comparing user identifying information maintained by the online system with user identifying information maintained by the third party system. To determine how accurately different types of user identifying information identify an online system user, types of user identifying information maintained by the online system are compared to types of user identifying information maintained by the third party system. A score is associated with various online system users based on the number of types of user identifying information associated with the user by the online system matching types of user identifying information associated with a third party system user. Based on the scores associated with different users, a measure of accuracy in identifying an online system user is determined for each type of user identifying information.