Turnover is a given at most startups, but the changes at Twitter have continued at a startling pace since it went public exactly one year ago. At least one of these departures—Rowghani's—has been attributed to clashes with Twitter CEO Dick Costolo. Others, at both the management and staff levels, have been chalked up to general confusion and frustration at the company. That's all unfortunate for Costolo, who was appointed as CEO in 2010 in part to stabilize Twitter's management after a fairly public power struggle between two of its founders.