I use stumbleupon to build traffic to my sites all the time. I exchange stumbles with other developers to get things going and then (hopefully) everything takes off from there. It's a great way to build traffic and get noticed, but I don't think it's the best for revenue
I certainly don't get a very high CTR from stumbleupon visitors but it really depends on the page they're landing on. If it's really interesting and the adsense ads are pertinent it doesn't seem to be too bad.
absolutely. I find stumble traffic to have extremely low conversion rates and practically 0 CTR for adsense. However it's good exposure and good for your alexa ranking
not only from SU, all traffic that came from social bookmarking sites have extremely low conversion rates,
If StumbleUpon -> Then serve flashy gimmick like flash game or cool picture -> Then you have a chance to make more than $1