One of Mark Zuckerberg's key lieutenants is returning to Facebook, shaking up the company at a pivotal time

· *Former top Facebook executive Chris Cox is rejoining the company as chief product officer, a year after leaving.*
· *His rejoining means a shake-up at the company's upper ranks, with multiple top executives now reporting to him instead of Mark Zuckerberg.*
· *Facebook has faced an employee revolt in recent weeks, and the...