I had never really factored Stumbleupon into my SEO strategy. I merely posted a couple bookmarks on the site, cuz why the hell not? A couple days ago, one of the bookmarks I posted went viral and boosted my traffic a ton. This traffic actually lead to CPA conversions. I can only imagine what it would do for AdSense sites. Of course, this method is rather inconsistent and relies a bit on luck, however, it can be very profitable, especially if you apply it to a ton of your sites. Keep in mind that this won't work for all niches. That being said, if your site is in any way fun, unique, interesting, or odd, this can bring in a ton of viral traffic. I know there are bookmarking tools out there, but I think that if you want to do this properly, you should do it manually. Choose the right pages from your site to bookmark and then choose your category and tags wisely. You can even create a page on your site that is designed to go viral in hopes that someone will explore the rest of your site. Anyway, give it a try and see if it can boost your traffic.