eBay has acquired StumbleUpon, a Web browsing recommendation site that mixes social networking and novel search functionality based on peer reviews, for US$75 million.

Founded in 2001 by three Canadian software engineers, StumbleUpon boasts over 2.3 million users who collectively recommend and rate Web sites, based on a simple "thumbs up" or "thumbs down" vote, to other registered users.

When installed in a browser, users can add sites to StumbleUpon's database of shared knowledge and discover sites of interest by simply clicking on the StumbleUpon button which randomly suggests a site based on the user's specified preferences. Users can then vote whether they like or dislike the site StumbleUpon has selected for more accurate returns the next time around.

Its innovative approach to Web search has been dubbed by many observers as a pioneer of Web 3.0, as it combines general internet search capability with a user's personal data and aggregated community data.

StumbleUpon is free for users and generates revenue through advertising. Members who pay an annual fee bypass advertisements completely.

In a message on eBay's website, Bill Cobb, eBay's North America president, said the online auction giant is "always exploring opportunities in new technologies and businesses --especially ones that pioneer new communities and instill trust."

"Our intent is to support the growth and evolution of this community-based business," he said. "Although there are no definite plans to share at this time, as the site evolves, we'll be exploring the possibilities for synergies between StumbleUpon and eBay marketplaces, Skype, and PayPal. We'll be sure to keep you posted."

Cobb said StumbleUpon would continue to run as a separate business unit within eBay while its founders would become eBay employees. eBay senior director, Michael Buhr, will head up the management, but so far no changes or added features have been announced for either site.