Yes, that bums me out as well. I guess each language market has its own opportunities and limitations.
If your site is bookmarked (or whatever StumbleUpon's equivalent is) that means the viewer saw something interesting about the site, hence they want to stay there (not click on your ads). So if you are a non-ecommerce site, a bookmark is mostly a way to get exposure and not money. But if you are an ecommerce, it should bring more eventual sales.
The traffic sucks. I know when I use stumble I click the stumble button like a madman flying through sites until something catches my eye.