Trouble for Twitter Thursday. Its chief operating officer Ali Rowghani has resigned after several months of watching his responsibilities get whittled away. In a filing, the social network said though Rowghani had resigned, he would remain a Twitter employee, acting as strategic adviser to CEO Dick Costolo. Of note, Twitter said it didn't intend to find a replacement but rather that Rowghaniâ¿¿s remaining responsibilities would be delegated to existing executives. Rowghani had held the role of COO since 2012, prior to which he had acted as chief financial officer for two years. Year to date, shares of the company have fallen more than 40%.