- StumbleUpon was founded in 2001 in Calgary by Anthony Kirlew and had 1 million registered users by 2006. It was acquired by eBay in 2007 for $75 million but was bought back by its founders and investors in 2009 for $26 million. It now has 10 million regular users and has indexed 1 million unique URLs. - StumbleUpon uses a paid advertising model with guaranteed visitors, targeting by demographics and interests, and a low cost per visit of $0.05 with no minimum spend or bidding required. - The site aims to provide personalized recommendations by learning from the sites users vote on with "thumbs up" or "thumbs down" as well as their stated interests and channels they follow