Stupid question? probably... but do you get any backlinks when you have stumbles on your site or does that just drive traffic and not create backlinks? thanks.
You'd better get a nofollow/dofollow plugin for Firefox and closely analyze the link structure on SU. Al.
only if there dofollow , but just to note that nofollow also help as it makes your links look more natural
Stumbleupon is mainly a traffic driver where there is usually a large spike of traffic when something is first stumbled followed by a steady to low stream afterwards.