Those who are dedicated users of StumbleUpon won’t be at all surprised by what I found via an interview on Friday. For those of you who haven’t used the service, or perhaps you have but you’ve not done so fully, read on. StumbleUpon is a true example of the power that a site can hold, even when it’s one that nobody really talks about.

First, a history lesson. StumbleUpon was created in 2001 as a better way to find really great photos online. The site’s founders simply wanted a way to do that, and created a Firefox extension that enabled it. Moving to the SF Bay area, a year later, StumbleUpon was acquired by Ebay for $75 million.

Fast forward a couple of years to 2009. The founders, with the help of outside financing, decided to buy StumbleUpon back from Ebay. The site releases the su.pr URL shortener, its web toolbar and mobile products and more recently a video product, too.

I had the chance to talk with Marc Leibowitz, StumbleUpon’s VP of Marketing and Business Development. We discussed where the site came from, where it’s going and what it has seen along the way.

Since parting from Ebay, Leibowitz tells me that StumbleUpon has grown from 6 million to 15 million users. Recommendations have tripled, now surpassing 800 million every month. Each month, over 3-4 million users are active on StumbleUpon, with the primary base being in the US.

StumbleUpon wants to be synonymous with discovery. It has done a great job of this when it comes to web content, but Leibowitz tells me that both products and places should be in the future. For the short term, continuing to improve both the mobile and video versions of SumbleUpon are the immediate plans.

And how will that growth happen? Where does StumbleUpon make its money? Via advertised pages. Yes, some of the pages that you stumble are indeed seen because of paid placement. However, StumbleUpon takes the quality of its stumbled content very seriously.

Advertised pages are presented via the same algorithm that dictates the rest of the content that you’ll stumble. A fixed amount of money is paid for every recommendation of an advertised page. Those same social and curated signals that show you the cool stuff you’re looking for are used to present targeted, relevant advertised pages.

Is it working? According to Leibowitz, advertised pages are given the “thumbs up” almost as much as organic results. Since an advertised page will only be shown to a user once, the content remains fresh and not overly presented. Advertisers are able to target via StumbleUpon’s mobile apps, as well, and over sixty thousand StumbleUpon advertisers have used the product thus far.

So what’s next? Leibowitz tells me that StumbleUpon’s mobile app is growing 40%, month over month, and has just crossed the 1 million install mark. Clearly, mobile content and mobile browsing is a direction that SU will push in the future. On top of that, a fresh $17 million Series B financing will help to build out SU’s core discovery technology, as well as different media sources, including interactive TV.

It’s no wonder that StumbleUpon has topped the race for sending the most traffic of any social media site.