The last few months, Stumble Upon traffic on my site has been in the 1000+ hits per day. It's died down this month for the first time in a long time. What I've noticed is since SU traffic died, my CPM increased and my adsense revenue has increased at least 5-6x. Expected?
Depends what google was thinking where the traffic on your site is coming from..If they think the traffic was of high quality, they woiuld have displayed the low bid ads.