When Facebook counts ad impressions, they charge only when the ad is seen by a real person. In other words, they measure ‘viewed’ impressions rather than ‘served’ impressions.
Served impression is counted as long as an ad appears somewhere on the page (such as below the fold), even if the user never sees it. Viewed impression gets counted when the ad is displayed on a user’s screen.
Facebook recognizes that an ad drives value to an advertiser only when viewed by a real user. Thus, they use only viewed impressions to measure ad delivery across various platforms.
The system would not count the ad if it is technically served, but not viewed. The same stands true for organic content posted by businesses.
Though many may argue that all ads that appear on the screen are not viewed, it is still better than charging for ads that are never seen at all.