1. A method comprising: receiving, by one or more computer systems, sponsor specifications designating one or more concept nodes in a social network and a temporal factor to apply to sponsored stories; monitoring, by one or more computer systems, an activity stream for upload by a user of the social network of one or more multimedia objects comprising one or more image objects associated with one or more of the concept nodes, respectively; upon finding a matching multimedia object, by one or more computer systems: determining, based at least in part on one or more privacy settings of the social network set by the user, one or more other users of the social network who are allowed to view the multimedia objects; and exporting the matching multimedia object to a sponsored-story system; calculating, for one or more sponsored stories, each sponsored story comprising at least part of one or more of the image objects corresponding to one or more of the exported multimedia objects, a score for the sponsored story based at least in part on a time the respective exported matching multimedia object was uploaded to the activity stream by the user and the temporal factor designated by the sponsor specifications; and sending, to a client system of one or more of the other users for display, one or more of the sponsored stories based at least in part on the calculated scores.