A question for coderdude: how well did the traffic from StumbleUpon perform? Did it almost entirely bounce straight away, or was there a lot of clicking through to other pages?
Out of 132,000 Stumblers the average pages viewed was 1.42 and the average time on the site was 46 seconds. The bounce rate was 80.93% but it's hard to tell how GA is calculating that since I have so many off-site links. All in all I wouldn't call SU traffic very "sticky" but some of it did convert well (became subscribers or shared the site in other channels).
I didn't know about paid SU... i'm going to give it a try with one of my projects, www.plansharing.com. I've tried Facebook ads and Google AdWords. Neither of which has been very good. Facebook REALY doesn't want to send you off site, and mostly the google ads were run on spam trap websites in my experience.
If SU leads to organic traffic too, that's an interesting alternative.