Instacart has tapped one of the top female executives from Facebook to be its next CEO ahead of a possible Wall Street debut. Fidji Simo, who served most recently as a vice president and head of the Facebook App, will replace Instacart's cofounder Apoorva Mehta as chief executive of the fast-growing grocery delivery service. The hire makes Simo the rare female CEO of a large on-demand delivery company, and one that is reportedly making moves to go public. In a letter to the Instacart community Thursday announcing the news, Mehta said Simo was "at Facebook during its transition from a private company to the public market and understands what it takes to lead and scale a transformational company."