You don't want to continuously purchase traffic via StumbleUpon that should never be your ultimate goal.
Instead, you should utilize the advertising services there to jumpstart your blog traffic with the goal being to encourage Stumblers to vote for you, which will in turn, push other visitors to your site, for free.
Some bloggers fail to do this and end up paying for traffic longer than they would need to, if they just focused on creating a useful blog.
Even though the advertising costs are lower than typical PPC campaigns, you can save a ton of cash if you make your blog worthy of a stumble.
Set your blog in motion and watch the traffic continue to pour in, day after day!
Catch Them Before The Next Stumble
Since stumblers are similar to television channel surfers, you might not be able to hold their attention for long, no matter how compelling or interesting your content may be.
To get the most out of their visit, offer a subscription form on your site where you offer a free report, eCourse, something - Anything - to encourage them to leave their information.
That way, you can work on growing your list as well as growing your site.
Your subscription box should be placed in a highly visible area of your blog. You should also utilize graphics, or compelling headlines to make sure it's noticed.
Take Baby Steps
Don't spend a ton of cash on your first StumbleUpon ad campaign, instead, test your ad copy with just a small spend. Once you see it working, you can increase your daily budget.
Be sure to pay attention to the available stats to determine how many people voted for your site and continuously tweak it for the best performance you can get out of it.
Testing isn't fun but it's important because you want to get the most votes you can.
Once you are consistently receiving votes, you will start receiving a flood of organic traffic and when that happens, be prepared to pause your campaign before you purchase traffic for nothing.
Things To Avoid
Some bloggers believe that sending Stumblers to the very front page of their blog is the best choice, when advertising with StumbleUpon.
I would suggest that instead; create a landing page specifically for each campaign. If Stumblers have to seek out solid content, they'll be gone before you know it, assuming it's a paid advertisement or sales pitch.
Solution:
Choose a thread or page that carefully relates to your specific group of visitors (using categories and demographics), and choose a post that you feel is absolutely compelling.
Make it spicy, entertaining, controversial, whatever you have to do to make that page stick and become 'viral'.
Also remember to add your sites to StumbleUpon by creating an 'alias' account and Stumbling onto your own pages, after first checking to see if they have already been Stumbled.