nomis5 - 8:48 am on Jan 29, 2012 (gmt 0)

User preference ads generate more income than ads based on page content, that's the unfortunate fact. A couple of ad serving companies are depending on that fact and doing extremely well finacially.

It's allowing some ad serving companies to outperform Adsense in specific areas and I think Google is changing Adsense to counter the threat.