Google is out with their bad ads advertising report for 2013. On their blog they reported:

Stopping more bad ads from fewer bad sources

We removed more than 350 million bad ads from our systems in 2013.  To put that in perspective, if someone looked at each of these for one second, it would take them more than ten years to see them all.  This was a significant increase from approximately 220 million ads removed in 2012.  This trend has been consistent in the last several years and we attribute it to several factors, including: the growth of online advertising overall and constant improvement of our detection systems.

The number of advertisers we disabled, however, dropped from over 850,000 in 2012 to more than 270,000 in 2013.  In part, we attribute this decline to scammers — counterfeiters, for example — being thwarted by our safety screens and searching for less-secure targets.