StumbleUpon's Mobile Ad Breakthrough

For Internet content providers, figuring out mobile advertising is the next great digital puzzle. From Google and Facebook to the New York Times, companies are struggling to devise new smartphone and tablet ads that strike advertisers as more effective than clunky banner displays and don’t alienate users. A solution may have arrived via an unexpected source: a 12-year-old, nearly forgotten aggregator of links.

StumbleUpon, one of the Web’s first major social-bookmarking services, has seen better days. Acquired by EBay in 2007, the company was bought back two years later by its co-founders and investors including Accel Partners, August Capital, and venture capitalist Ram Shriram. Traffic to StumbleUpon’s collection of links, which users can vote up or down, has fallen by more than half, to 3.3 million monthly readers in the last two years, comScore data show. That’s far below the 9 million of category leader Reddit, closer to faded rivals such as Delicious and Digg.