1. A method comprising, by a server of a social-networking system: receiving, from a client system of a first user of the social-networking system, an indication of a reward-generating event, wherein the reward-generating event comprises the first user liking, sharing, commenting on, or posting a content object via an interface associated with the social-networking system; updating, by the server of the social-networking system, a social graph of the social-networking system based on the reward-generating event, wherein the social graph comprises a plurality of nodes and a plurality of edges connecting the nodes, each node corresponding to a user or content object associated with the social-networking system, wherein the first user corresponds to a first node of the plurality of nodes, and wherein updating the social graph comprises one or more of: adding a new node to the social graph corresponding to a content object posted by the first user; or adding an edge connecting the first node to a second node of the plurality of nodes corresponding to the content object liked, shared, or commented on by the first user; searching, by the server of the social-networking system, an event log of reward-generating events stored on the server to determine whether the update to the social graph matches a reward-generating rule defined by the social-networking system; responsive to determining that the update to the social graph based on the reward-generating event performed by the first user matches the reward-generating rule, allocating, by the server of the social-networking system, a reward to an account associated with the first user, wherein the reward is calculated based on the reward-generating rule; and responsive to determining that the update to the social graph based on the reward-generating event performed by the first user matches the reward-generating rule, sending, to the client system of the first user, a selected advertisement for presentation to the first user, the selection of the advertisement being based at least in part on a quality score for the advertisement, wherein the quality score for the advertisement is calculated based on a value associated with the update to the social graph.