  Targeted advertising remains an important part of the free web browsing
experience, where advertisers' targeting and personalization algorithms
together find the most relevant audience for millions of ads every day.
However, given the wide use of advertising, this also enables using ads as a
vehicle for problematic content, such as scams or clickbait. Recent work that
explores people's sentiments toward online ads, and the impacts of these ads on
people's online experiences, has found evidence that online ads can indeed be
problematic. Further, there is the potential for personalization to aid the
delivery of such ads, even when the advertiser targets with low specificity. In
this paper, we study Facebook -- one of the internet's largest ad platforms --
and investigate key gaps in our understanding of problematic online
advertising: (a) What categories of ads do people find problematic? (b) Are
there disparities in the distribution of problematic ads to viewers? and if so,
(c) Who is responsible -- advertisers or advertising platforms? To answer these
questions, we empirically measure a diverse sample of user experiences with
Facebook ads via a 3-month longitudinal panel. We categorize over 32,000 ads
collected from this panel ($n=132$); and survey participants' sentiments toward
their own ads to identify four categories of problematic ads. Statistically
modeling the distribution of problematic ads across demographics, we find that
older people and minority groups are especially likely to be shown such ads.
Further, given that 22% of problematic ads had no specific targeting from
advertisers, we infer that ad delivery algorithms (advertising platforms
themselves) played a significant role in the biased distribution of these ads.
