The over-supply of mobile ad inventory

"For publishers, the biggest challenge around monetizing mobile audiences is still the matter of inventory oversupply. The number of mobile apps and sites is growing rapidly, but advertiser investment continues to lag, meaning even networks are struggling to fill the majority of impressions they’re handed.
According to data from publisher-side ad management platform Smaato, that problem is only getting worse. During the third quarter of the year the firm said only 18 percent of impressions were filled by the top 20 U.S. mobile ad networks, representing a decline from the 19 percent of ad slots they filled between April and June."

Source:  DigiDay, 30th November 2012