Advertising networks pool advertising space at large numbers of websites and then offer this space to advertisers as part of targeted click packages. These networks have been growing dramatically and enjoying great success as advertises struggle to maximize the amount of activity they get for a given campaign.

The New York Times reports and offers an interesting travel market case study that suggests advertisers are shifting their focus from campaigns with larger publishers at higher cost to campaigns where they can maximize the number of clicks per dollar spent.

Unfortunately for advertisers the equations grow increasingly complex after allowances are made for click fraud and inferior targeting. My own experiences with per click campaigns have left me very skeptical that positive ROI is easy to obtain in all but extremely well managed campaigns with high margins on sales of the advertised item. Minimum bidding levels on many potentially valuable search terms - for example $0.30 at Google for terms relating to “hotels” - make positive ROI for popular terms even more elusive.