30% of ad impressions aren't seen

"Nearly 6 trillion display ad impressions were delivered across the web in 2012 as brand marketers have become increasingly comfortable with a medium capable of delivering strong marketing ROI. Despite delivering so many impressions, comScore research showed that an average of 3 in 10 ads are never rendered in-view, leading to significant waste, weaker campaign performance and a glut of poor-performing inventory that imbalances the supply-and-demand equation and depresses CPMs. Through the continued adoption of a viewable impressions standard, the market is beginning to embrace a digital scarcity model that better aligns monetization with the value created by the inventory."
Source:  Press release from comScore, 14th February 2013
Note - comScore made the same point last year - see my comment