One of HTC's top execs, Chief Operating Officer Matthew Costello, will depart after three years on the job, according to a report from Bloomberg. He'll join recent departees like Asia CEO Lennard Hoornik and others in leaving the company, which has seen unprecedented turnover of late amid its falling fortunes. His position will be filled for now by engineering president Fred Liu, who told employees via email that Costello would move to Europe and stay on as an executive adviser. Ironically, the latest departure comes in the wake of very good news for HTC, which just reported $970 million in May revenue -- nearly double what it took in last month and just shy of May 2012 figures. That can likely be chalked up to sales of the One, which is now being churned out at full speed (and soon in a stock Android version) but whether the fortunes of HTC's star handset can halt the brain-drain remains to be seen.