Have you any experience regarding stumble upon that how much traffic you got through this bookmarking site. i also wanna make experiments
The way stumble upon is designed. A page comes randomly (ok according to your selected category) , you view the homepage and you might not go to any other page and hit that stumble again (and visit another site randomly). That means visitors are leaving the site from the entrance and contributing in high bounce rate. The visitors are not that targeted. So they easily bounces of your page. hth