Why is this shocking?
Good question! There are some companies like Amazon where people above a certain level rarely leave, even the founders/CEOs of the companies they acquire stick around.
Until this year, the perception was Facebook was one of those golden companies that could attract any star like Cheryl Sandberg and keep her for a decade or more. The two Instagram founders were on top of the star list and had continuously talked about how happy they were, and how they could not have enjoyed the level of success they have without the acquisition.
Far as I know, Zuckerberg has always been rated incredibly high on Glassdoor by Facebook employees.