Troubled Groupon replaces CEO Eric Lefkofsky with Rich Williams

• Former COO Williams must tackle downward spiral of margins
• Former e-commerce poster child announces $150m-$200m marketing blitz

Groupon said on Tuesday evening its co-founder Eric Lefkofsky would step down as CEO and would be replaced by its chief operating officer, Rich Williams.

One of Williams's challenges will be to boost margins, which have been on a downward spiral. Gross margin in the third quarter was about 46% - a far cry from more than 80% in 2011.