31% of US online ad impressions aren't seen

"comScore, Inc, a leader in measuring the digital world, today released full study results from its U.S.-based vCE Charter Study involving online advertising campaigns for 12 premium national advertisers. The study found that, in many cases, a large portion of ad impressions are not delivered according to plan, and that the quality of ad delivery can vary greatly based on a variety of factors, including site, placement, creative and targeting strategy. The study evaluated ad delivery based on a several key dimensions, including whether or not the ads were delivered in-view, to the right audience, in the right geography, in brand safe environments and absent of fraud. The complete results are included in a complimentary white paper, which can be downloaded at the following link: http://www.comscore.com/vce-charter-study
[...]
The vCE Charter Study includes a variety of detailed findings that shed light on the current state of online ad delivery and its implications for different participants in the online advertising market. Key findings include:
In-View Rates are Eye-Opening. The study showed that 31% of ads were not in-view, meaning they never had an opportunity to be seen. There was also great variation across sites where the campaigns ran, with in-view rates ranging from 7% to 100% on a given site. This variance illustrates that even for major advertisers making premium buys there is a lot of room for improvement.
Targeting Audiences Beyond Demographics Can be Powerful. Generally, campaigns that had very basic demographic targeting objectives performed well with regard to hitting those targets. For example, those with an objective of reaching people in a particular broad age range did so with 70% of their impressions. Predictably, as additional demographic variables were added to the targeting criteria (e.g. income + gender), accuracy rates of the ad delivery declined. However, the results also showed that 37% of all impressions were delivered to audiences with behavioral profiles that were relevant to the brand (i.e. consumers with demonstrated interests in categories, such as food, auto or sports). One campaign had 67% of its impressions viewed by the target behavioral segment.
The Content in Which An Ad Runs Can Create Problems for Any Brand. Of the campaigns analyzed, 72% had at least some impressions that were delivered adjacent to objectionable content. While this did not translate to a large number of impressions on an absolute basis (141,000 impressions across 980 domains), it is important to note that 92,000 people were exposed to these impressions. This demonstrates that brand safety should be of concern to all advertisers.
Fraud is the Elephant in the Digital Room. Fraud is an undeniably large and growing problem in digital advertising. The results showed that an average of 0.16% of impressions across all campaigns was delivered to non-human agents from the IAB spiders & bots list. Although this percentage might appear negligible, there are two important considerations to keep in mind. Only the most basic forms of inappropriate delivery were addressed in this study. When additional, more sophisticated types of fraud are considered, the problem will only get larger. Like brand safety, fraud should be an important concern for all advertisers.
Digital Ad Economics: The Good Guys Aren't Necessarily Winning. The study showed that there was little to no correlation between CPM and value being delivered to the advertiser. For example, ad placements with strong in-view rates are not getting higher CPMs than placements with low in-view rates. Similarly, ads that are doing well at delivering to a primary demographic target are not receiving more value than those that are not. In other words, neither ad visibility nor the quality of the audience reached is currently reflected in the economics of digital advertising."
Source:  Press release from comScore, 26th March 2012
Note - comScore released similar research earlier in the year.  I'll repeat the note I made at the time:
While this research is valid, it's worth mentioning the different ad trading models.  Most ads are not bought on a 'cost per view' (CPM) model, but are bought on a cost per click (CPC) or cost per action (CPA) basis.  This means that the advertisers for these campaigns don't pay for the ads until there is a click or a specified action - like a sale or a registration - so they are not being charge anything for ads that aren't seen. 
Part of the job of the agency is to ensure that the ads bought on a CPM basis appear in good positions - that is 'in view' making the site push the other ads down to the bottom of the page.  Similarly with ads appearing in unsafe content - part of the job of the agency is to make sure that this does not happen.