SO MANY.
Again, the glamor of working at Facebook was enough, for a while. And when the company and research team were smaller, it was a lot better.
But now, the research culture at Facebook is *TOXIC*. Management is full of people who are there because they want to further their careers, not because they want to help their team members grow. I had a string of awful managers, one of whom actively discouraged me from following my ambitions. (Note that not all managers are awful, some were amazing and I sought them out as mentors instead of my own managers.)
Being a researcher at Facebook - especially from 2015 on, was discouraging and miserable enough that it ruined my confidence in my own abilities. Researchers were competitive with each other, and instead of focusing on improving the product, focused on edging out their colleagues for opportunities and recognition.
I know several wonderful people who left the company because of the toxic atmosphere and poor management.