The recent penetration of smart mobile devices into the consumer market sets a stage for novel network applications. In particular, we envision a paradigm shift in the commercial advertising model facilitated by the widespread uses of these devices: advertisements circulate in a word-of-mouth fashion among device users and reach potential customers based on the users' knowledge about their contacts. In this paper, we identify two major challenges baffling the deployment of such an application: users' selfishness and their privacy concerns. We address the selfishness issue by proposing an incentive scheme which aligns users' interest with that of advertisers in a way that the users are willing to fully explore their social knowledge for effective advertisement deliveries — the emphasis is not only on users' participation but also on the extent and effectiveness of their contributions. We address the privacy concerns by designing a privacy-preserving evidence-collection mechanism, on which the incentive scheme is based. In addition, our design is 1) appealing to advertisers by guaranteeing effectiveness and controllability of the incentive dispensing and 2) robust against users' misbehaviors. We perceive incentive and enforcement as the keys to unlock the power of users' collective intelligence for effective information dissemination.