By bounce, I am referring to bounce rate. A bounce is recorded when a visitor views only one page of your site during the visit. As someone who runs a blog, I strive to make my content coherent (ie. my blog contains a lot of programming and technology content) so that it appeals to a certain audience. A good indicator of whether my blog is coherent is if people view more than one page per visit on average, or if I have a low bounce rate.
People usually find my blog through search, and so they land directly on a post (instead of the front page). Likewise, when my blog get Stumbled, the landing page is always a post. When visitors finish reading that post, ideally they’d take a look around to find more of the same content. With that in mind, would the bounce rate of visitors from Google and StumbleUpon be the same? Here is my referral data from the last two months (July 1st - Sept 11th):

As you can see, the bounce rate from StumbleUpon visitors is very low compared to visitors from my other top referrals.
My conclusion (well, it’s more or less an assumption, but an educated one) is that the StumbleUpon Interests selection works very well to channel interested readers to your content. It’s not a bad bet that users who go through the entire process of signing up to StumbleUpon, installing the tool bar, and setting up their interests must really be passionate about the selected topics.
That or they have a lot of time to kill, which, admittedly, is something that StumbleUpon also does very well.
The moral of the story? Tag your Stumbles wisely!
P.S. If you have a blog which gets Stumbled often, post some of your findings on your bounce rate!