I always stumble my articles, and have found StumbleUpon to consistently bring 100 or say visits a day. Imagine my surprise when today I check analytics and find 2000 visitors referred from there. The site is at www.thelingerieblog.com, which I can't find anywhere on Stumbleupon. Might I have been featured or something?
For me I get an average of about 50-100 visitors from stumbleupon. Not great but at least it is something.
Might be your site were reviewed by someone other stumblers too. And, its great having lots of traffic for your site but traffic from SU is very limited, not regular.