  Data collection and aggregation by online services happens to an extent that
is often beyond awareness and comprehension of its users. Transparency tools
become crucial to inform people, though it is unclear how well they work. To
investigate this matter, we conducted a user study focusing on Facebook, which
has recently released the "Off-Facebook Activity" transparency dashboard that
informs about personal data collection from third parties. We exposed a group
of n = 100 participants to the dashboard and surveyed their level of awareness
and reactions to understand how transparency impacts users' privacy attitudes
and intended behavior. Our participants were surprised about the massive amount
of collected data, became significantly less comfortable with data collection,
and more likely to take protective measures. Collaterally, we observed that
current consent schemes are inadequate. Based on the survey findings, we make
recommendations for more usable transparency and highlight the need to raise
awareness about transparency tools and to provide easily actionable privacy
controls.
