On a related note, I read you can buy "stumbles" and readers from StumbleUpon and Reddit, but it sounds like DuckDuckGo and Whiteboard had mixed results: http://tinycomb.com/2010/07/19/our-reddit-ad-by-the-numbers-...
Instead try using http://www.stumbleupon.com/url/www.yourwebsite.com/
You'll be able to see the review page that people are stumbling from. One one of my websites receives average 200 unique free stumbles per day and I have since become a paid advertiser, though their categories for targeting are very limited, imho.
What kind of results have you been getting with paid ads?
I looked at the "referring sites" (in google analytics) and found that one of the pages was /refer.php which was the advertising page you mention.
I have an unproven feeling that my analytics from SU were somehow artificially inflated or something like that. My own personal experience is that I'm unimpressed with SU.
Others would disagree with me, and hopefully, someone somewhere will be able to convince me otherwise.
In general, I'm not a fan of paid search since it creates an artificial boost in traffic. It's fine if you're a large company with a big ad budget, but even so, as soon as you turn of the spigit, the water runs dry...