emarketer reporting:
Brand advertisers are accustomed to paying for views—both on TV and online. In the digital world, impressions have long served as a proxy for views, but findings from comScore and ad verification solutions company AdSafe Media found that paying for raw impressions is costing brand advertisers money not correlated to effectiveness.
To better understand the true number of display ad impressions seen by internet users, comScore analyzed the display ad campaigns of 12 major brand advertisers. The study found that an average of just 69% of ads were in-view. By comScore’s definition, to be in-view, a user had to have seen 50% of an ad’s pixels for at least 1 second.
The larger the website, the greater the percentage of in-view display ads. The top 50 sites for each brand’s industry vertical, as classified by comScore’s MediaMetrix ratings, had the highest in-view percentage (77%). That number declined when broadening to the top 100 sites (74%) and top 500 sites (70%).

The fact that larger publishers’ sites tend to have greater quantity and variety of premium ad inventory space could be one influencing factor. And additional data from AdSafe Media showed publisher sites had a greater percentage of in-view ads than ad networks and exchanges...
http://www.emarketer.com/Article.aspx?R=1008965&ecid=a6506033675d47f881651943c21c5ed4