FB interview standards are extremely high. People who work there, probably about 95% of them could work at any other tech company if they choose to. What keeps a majority of the people there I would assume is 2 things,

#1. The pay is very close to top in the industry, hedgefunds/netflix pay usually more.

#2. Getting to work on products/features that are used by millions and sometimes billions of users, not many companies can offer that to a candidate.

I personally am hounded by their recruiters but will never work there due to moral objections about their leadership and their companies effect on society. That being said I am not judging people for working there, everyone has a free choice to make and some people may not have my convictions and there are many great people who do work there.