There is a great interview, Q&A with Garrett Camp on SELand, one of the founders, on some of the technology:
Our 2 million registered users stumble around 5 million times a day, so we have a pretty active user base. If they find something new, it's incredibly easy for them to submit it to us. All they need to do is click the thumbs-up button on the toolbar and it's submitted to our database. We get over 16,000 new URL submissions a day - all new and unique content endorsed by our members... We have a classification engine which automatically places content into one of 500 predefined categories based upon on-the-page factors. This means most content submitted can be distributed to interested members even before tags have been applied.It is also a great relatively undervalued marketing channel:
StumbleUpon has a unique business model that works well for marketers where we can deliver traffic directly to your site. You can target by category, age, gender and location. So for product launches, distributing audio/visual content or just getting feedback on your blog, StumbleUpon often works better than PPC approaches since targeting is precise and no click through is required.
Perhaps eBay will extend it to products... or videos of products. Who knows. I'm not sure I get this one...
In response, Google has launched it's own blatant rip-off. See Google's blog post: "Searching Without a Query. Google's new personalization will take into account not only your search history, but now also your web browsing history via the Google Toolbar (in a separate post Your Slice of the Web). You did know that the Google Toolbar tracked the sites you visit if you had PageRank turned on, right?
Remember my previous post from Hakia's future of search-- How much do you trust Google with your data (mail, docs, purchase history (Checkout), search history, web browsing history, files (GDrive), etc...) ? Imagine the possiblities... for good or evil.