The key to StumbleUpon is that its users can share interesting Web content. When they find something they like online, they have a Web-based outlet through which they can let others know about the site. Other users can then view that content, through topic-orientated browsing or purposeful searching, and reciprocate the favor by sharing their own interesting finds.
In this way stumbleupon share the interesting content and do viewing boost to your content.
Know more:
http://computer.howstuffworks.com/in...tumbleupon.htm