I am just not grasping Stumble Upon. I downloaded the stumble upon toolbar. That's it. What am I supposed to be doing with it? LOL
I get a good amount of hits from Twitter, as well as sites like Mom Bloggers Club, Kirtsy, etc. I find that if you can get a blog post featured on a site like digg or kirtsy, you can get a real boost in hits. Of course, some people just breeze in and never come back, but it's worth a shot.
I definitely receive the most amount of traffic from StumbleUpon - especially if I've had a recently submitted article. But despite this, StumbleUpon is always in my top 5 list of referrals (it's currently the 2nd most popular way that people get to my site/blog - with direct input being #1, so technically it's my biggest source of referral traffic, at least at the moment.)
Twitter is the 2nd highest social network - it's in my top 10 referral list at the moment