We love StumbleUpon here on Martech Zone. In fact, it's often our number one referral source. It's also been the number one referral source of traffic on the web! StumbleUpon walks through some other advantages of its service here in this infographic – including the fact that the time that your link continues to refer visits is much longer than sites like Facebook or Twitter. Facebook and Twitter are an ongoing stream… links come and go. Since StumbleUpon is not based on time but on popularity, a well written post, or an infographic ;), will continue to drive traffic… sometimes for months.
If you don't have a StumbleUpon badge for your site, you should absolutely think about adding one today. I would add that visually enhanced posts… images, video and infographics, do much better in getting stumbled than textual posts.