  Ad targeting is getting more powerful with introduction of new tools, such as
Custom Audiences, behavioral targeting, and Audience Insights. Although this is
beneficial for businesses as it enables people to receive more relevant
advertising, the power of the tools has downsides. In this paper, we focus on
three downsides: privacy violations, microtargeting (i.e., the ability to reach
a specific individual or individuals without their explicit knowledge that they
are the only ones an ad reaches) and ease of reaching marginalized groups.
Using Facebook's ad system as a case study, we demonstrate the feasibility of
such downsides. We then discuss Facebook's response to our responsible
disclosures of the findings and call for additional policy, science, and
engineering work to protect consumers in the rapidly evolving ecosystem of ad
targeting.
