From AdAge in an article entitled: "ComScore Study Confirms What We Already Knew: You're Wasting Money on Ads No One Sees"

One of the key findings: 31% of the 1.7 billion ad impressions were never in view. The number is probably not a shock to many in the space, and is the main reason why so-called remnant inventory sells for a fraction of the space above the fold. But with this knowledge, advertisers can more accurately manage expectations and calculate ROI, said ComScore CEO Magid Abraham. 

Not a huge surprise- online display ads have always produced very poor results. Full story here