This chapter addresses the problem of architecting display ad networks for online social media. The basic question behind this work is: Can display advertising in social media be effective while providing rigorous privacy guarantees? The chapter exposes the problems that display advertising faces in social media, introduces a display ad-network architecture organized around the goals of effectiveness and privacy enforcement, and describes a type of social media for which the architecture is ideally suited. To deliver high effectiveness in a social media environment, the ad network must function as an embedded component of the environment with ambient intelligence. Moreover, the architecture must constrain data and mechanisms in order to deliver rigorous privacy guarantees as a baseline and an additional set of choices to enforce even stricter views on privacy. Ambient social media sites, described later in the chapter, are the most appropriate form of social media for the proposed architecture.