On Tuesday morning, Facebook employees were quiet even for Facebook employees, buried in the news on their phones as they shuffled to a meeting in one of the largest cafeterias at the company’s headquarters in Menlo Park, Calif. Mark Zuckerberg, their chief executive officer, had always told them Facebook Inc.’s growth was good for the world. Sheryl Sandberg, their chief operating officer, had preached the importance of openness. Neither appeared in the cafeteria on Tuesday.