Your earnings should have stayed constant...the extra traffic should make ~$0 while the usual traffic should act as usual.
StumbleUpOn Traffic = No More Money Maybe 10,000 Visitors and 2 clicks Stumble is using NoFollow tag on external links
in december thanks to stumbleupon i made 9 amazon affiliate sales. I tracked the links and traffic in amazon, and it showcased that influx of traffic.
Stumble is great. It is actually the only usefull service of the kind. Digg etc is all about gettin traffic. No fun there. Imo Stumble = quality, Digg = quantity. People from digg click more ads tho.
i get huge traffic on my tuning website from stumbles... like, over 1000 uniques daily but too few click on the ads