According to a 2009 report by the Internet Advertising Revenue, 22.7 billion dollars have been spent for online advertising. Twenty two percent of this figure is for display ads. These are huge numbers, but there are very few studies about how effective these ads are and the huge amount of money spent on them. This is the summary of an article by Randall A. Lewis, David H. Reiley, and Taylor A. Schreiner. You can get the pdf of the behavioral targeting article here: Ad Attributes and Attribution: Large-Scale Field Experiments Measure Online Customer Acquisition.