In economics any time there is an abundance of supply the price for that particular good goes down. The easier to obtain, the less ‘valuable’ that resource is. In today’s online world there is no shortage of traditional Internet measures like ad impressions because of the huge amount of traffic being run through social networks.
The trouble is that another traditional measure for advertising, cost per thousand or CPM then gets taken down with it. Of course, not all sites on the Internet are created equal when it comes to what kind of ads are served and how they are received but when clumped together, social network traffic is the high volume low cost drag on the overall market pricing. AdAge reports (with a hat tip to paidContent)