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.