Zynga CEO Mark Pincus speaks at the Zynga Unleashed event in San FranciscoBy Tim McLaughlin | BOSTON Zynga Inc Chief Executive Mark Pincus said the social games company can double the number of its paying players but he and other executives sidestepped questions about player retention and churn rates, speaking at a luncheon ahead of its IPO. Zynga executives on Thursday fielded more than a ...