Move over Google, here comes Stumbleupon! Well, that may be going too far at this point. But this five year-old, privately held social search company, which has flown under the radar for the most part, is starting to make some noise. You must experience the browser plug-in tool for yourself in order to grasp how cool it really is. Stumbleupon recommends popular Web sites to its users based on their preferences - like astrology, sports or China - and the endorsement of others in the Stumbleupon community within those categories. The toolbar, which work best in Mozilla Firefox, works like a remote control that allows you to channel surf cool new sites that are up your alley.

Stumbleupon has over 2 million registered users, primarily from Mozilla Firefox, and recently launched a public version for Microsoft’s Internet Explorer, which should expand its membership significantly.

I recently started using Stumbleupon, so I don’t know all the ins and outs yet. One great feature is how the program adapts to your preferences, as you rate sites over time, constantly honing in on your taste. For example, I chose “Atronomy” as an interest, and when I clicked on the “Stumble” button in my browser toolbar, I landed on a site with a lot of imagery, awesome pictures from the Hubble Space Telescope, that interested me very much, so I clicked on the Thumbs Up “I like it” icon. I had also selected “Photography” as an interest, so perhaps the algorithm brought up images.

Here is another cool features that I discovered already. Check out this Google search results page below, in a keyword search “stock trading”. If you choose the option, Stumbleupon annotates each result with star ratings and site reviews from its users. Very cool!

Is this the future of search? Tagging sites, like Flickr, digg and Del.ic.ious are proving the power in giving power to the people through user-generated communities, social bookmarking and in-depth customization, hallmark features of the open-source movement, as opposed to proprietary, control-oriented products like those of Microsoft. Now Google faces a challenge in how to respond to innovation from smaller, niftier players like Stumbleupon. Once the cool, new kid on the block, Google has grown up, and must deal with such innovation by keeping up with the pace in their own R&D efforts, buying out their competition, as they did with YouTube, or just focusing on what they do best and not trying to be Jack of All Trades.

We’ll see how it plays out. In the meantime, check out Stumbleupon for yourself for a unique experience. I encourage you to join the growing community, and if you do, feel free to add me to your Friend List (user name is “arousta”). My good friend, David Richeson, author of 360 Degree Success, prompted me to join, and now I’m passing that invitation on to you all. Happy “stumbling” as they say.

Next important question: how to start optimizing your web properties for social search sites like Stumbleupon? The answer will be addressed in an upcoming post.