An online system tracks identities of users that interact with the online system. The online system sends a browser identifier for storing on a client device that interacts with the online system. The browser identifier uniquely identifies a browser of the client device used for interacting with the online system. A content provider system receives the browser identifier from the client device and uses the browser identifier for logging user actions associated with content provided by the content provider system. The content provider system sends user action logs to the online system and the online system determines users that used the client device at a timestamp associated with the user action log. The online system provides the user identifiers to the content provider system. The content provider system uses the user identifiers to generate reports.