Dynamic enforcement of privacy settings by a social networking system on information shared with an external system

An external system (such as a website) that interacts with users communicates with a social networking system to access information about the users, who may also be users of the social networking system. If a privacy setting is changed in the social networking system, and the change applies to information that has been shared with an external system, the change is enforced at the external system. For example, the external system may be notified that the information is invalid and must be deleted, or the external system may periodically request the information so that changes to the privacy settings are eventually experienced at the external systems. When an external system again needs the information, whether expired naturally or actively invalidated by the social network, the external system sends a new request for the information, which is subject to the (possibly revised) privacy settings.