  Online advertising is the major source of income for a large portion of
Internet Services. There exists a body of literature aiming at optimizing ads
engagement, understanding the privacy and ethical implications of online
advertising, etc. However, to the best of our knowledge, no previous work
analyses at large scale the exposure of real users to online advertising. This
paper performs a comprehensive analysis of the exposure of users to ads and
advertisers using a dataset including more than 7M ads from 140K unique
advertisers delivered to more than 5K users that was collected between October
2016 and May 2018. The study focuses on Facebook, which is the second largest
advertising platform only to Google in terms of revenue, and accounts for more
than 2.2B monthly active users. Our analysis reveals that Facebook users are
exposed (in median) to 70 ads per week, which come from 12 advertisers. Ads
represent between 10% and 15% of all the information received in users'
newsfeed. A small increment of 1% in the portion of ads in the newsfeed could
roughly represent a revenue increase of 8.17M USD per week for Facebook.
Finally, we also reveal that Facebook users are overprofiled since in the best
case only 22.76% of the interests Facebook assigns to users for advertising
purpose are actually related to the ads those users receive.
