  Every day, millions of users reveal their interests on Facebook, which are
then monetized via targeted advertisement marketing campaigns. In this paper,
we explore the use of demographically rich Facebook Ads audience estimates for
tracking non-communicable diseases around the world. Across 47 countries, we
compute the audiences of marker interests, and evaluate their potential in
tracking health conditions associated with tobacco use, obesity, and diabetes,
compared to the performance of placebo interests. Despite its huge potential,
we find that, for modeling prevalence of health conditions across countries,
differences in these interest audiences are only weakly indicative of the
corresponding prevalence rates. Within the countries, however, our approach
provides interesting insights on trends of health awareness across demographic
groups. Finally, we provide a temporal error analysis to expose the potential
pitfalls of using Facebook's Marketing API as a black box.
