Garrett Camp, founder of StumbleUpon, is stepping down as active CEO, Business Insider is reporting. He will stay on as the company chairman, but the abrupt resignation leaves StumbleUpon suddenly without a CEO. They are now conducting a public search for a new one.

Mr. Camp stuck with StumbleUpon for 10 years, after creating it as a grad student. In 2007 he sold it to eBay for a reported $75 million, only to buy it back a year later. After the reacquisition he grew the company from 5 million to 25 million users.

StumbleUpon bills itself as a “social discovery” site, where users share links to interesting sites within a given field. Bloggers and news organizations are fond of the site because it can help drive traffic when covering a particular topic. StumbleUpon users get to “stumble” their interests, finding new information and entertainment they would have otherwise missed.

The question right now is who will be his replacement? The San Francisco Chronicle already has a few ideas, listing three of their top picks soon after he stepped down yesterday. They think long time Google executive Marissa Mayer, former CEO of Digg, Jay Adelson, and former COO of Groupon, Rob Solomon would all make a good impression. Of these choices I think Jay Adelson would have to be at the top of the list. As the former CEO of another popular link-sharing site, he has the most direct experience in the realm of social discovery.

According to Business insider, StumbleUpon made a conscious effort to make the CEO search public. Keeping the information private may have lead to a leak and rumors of board discontent with Garrett. Making it widely known also gives them an opportunity to find a broader range of candidates. They are seeking someone with international experience, as they plan to launch local versions of StumbleUpon in Europe this summer.

In a company blog post, Garrett is humble and thankful to the people who made StumbleUpon what it is today: “The last 10 years have been an amazing ride, and I’d like to thank everyone who has made this possible. It has been the collective effort of hundreds of employees, investors, advisors and partners who have helped StumbleUpon get where it is today. And most of all, I’d like to thank our users – without our community none of this could have been possible. Thank you for helping us curate the web, and sharing over 35 billion stumbles to date.”

Garrett has also been offering some philosophical advice on Twitter. Understandable considering the huge step he has just taken.