1. A method comprising: receiving, by one or more computer systems, a multimedia object from a first user of an online social network, wherein the first user is represented by a first user node of a social graph associated with the online social network; detecting, by one or more computer systems, one or more concepts associated with the multimedia object, the one or more concepts being represented by one or more concept nodes of the social graph, respectively; and identifying, by one or more computer systems, one or more second users of the online social network as being exposed to the one or more concepts associated with the multimedia object, each second user being identified based at least in part on one or more social actions associated with the second user with respect to the multimedia object, wherein the one or more second users are represented by one or more second user nodes of the social graph, respectively, each second user node being within a specified degree of separation of the first user node.