I have used stumbleupon to drive traffic to a photoblog ( via stumbles, not advertising ).
If you can get some people to stumble your stuff you get traffic, 100s/1000s in 1/2 days and after that it goes back to zero, with a couple traffic spikes/year when some drunk stumbler hits that thumb up button again. At this point you get few hundreds hits. Stop.
This is what you can expect with a photoblog, btw that photoblog is collecting dust at the moment, maybe doing it regularly would bring some returning users, I don't know.
You can assume that most stumblers have the attention level you'd have while eating and having a single hand to obsessively click that 'stumble' button.
Probably there's a way to make it work, maybe advertising something 2.0 or some VERY ATTRACTIVE ( and fast loading ) flash landing page where you collect an email address?
Just a couple ideas.