Facebook has added two more board members, ending a period of unusually high turnover for the social network’s group of directors. The new appointments are Nancy Killefer, a former partner at McKinsey and ex-CFO for the U.S. Treasury, and Tracey T. Travis, the CFO of Estée Lauder. Both are filling board positions lost through a string of departures last year that included Susan Desmond-Hellmann,...