He's being replaced by Executive Chairman Eric Lefkofsky and Vice Chairman Ted Leonisis. They will serve as co-CEOs until they find a permanent replacement.

Here is Lefkofsky's statment: On behalf of the entire Groupon Board, I want to thank Andrew for his leadership, his creativity and his deep loyalty to Groupon. As a founder, Andrew helped invent the daily deals space, leading Groupon to become one of the fastest growing companies in history. Groupon will continue to invest in growth, and we are confident that with our deep management team and market-leading position, the company is well positioned for the future.

Source: http://techcrunch.com/2013/02/28/ceo-andrew-mason-replaced-b...