  Targeted advertising is meant to improve the efficiency of matching
advertisers to their customers. However, targeted advertising can also be
abused by malicious advertisers to efficiently reach people susceptible to
false stories, stoke grievances, and incite social conflict. Since targeted ads
are not seen by non-targeted and non-vulnerable people, malicious ads are
likely to go unreported and their effects undetected. This work examines a
specific case of malicious advertising, exploring the extent to which political
ads from the Russian Intelligence Research Agency (IRA) run prior to 2016 U.S.
elections exploited Facebook's targeted advertising infrastructure to
efficiently target ads on divisive or polarizing topics (e.g., immigration,
race-based policing) at vulnerable sub-populations. In particular, we do the
following: (a) We conduct U.S. census-representative surveys to characterize
how users with different political ideologies report, approve, and perceive
truth in the content of the IRA ads. Our surveys show that many ads are
"divisive": they elicit very different reactions from people belonging to
different socially salient groups. (b) We characterize how these divisive ads
are targeted to sub-populations that feel particularly aggrieved by the status
quo. Our findings support existing calls for greater transparency of content
and targeting of political ads. (c) We particularly focus on how the Facebook
ad API facilitates such targeting. We show how the enormous amount of personal
data Facebook aggregates about users and makes available to advertisers enables
such malicious targeting.
