the max offer I have seen is 160k salary + 300k/4 + 100k signing
It is lower than many companies. So when someone has many good offers it is less likely to choose fb.
Minus 100k sign on. It’s only for extreme/exceptional cases or new grads/returning interns. Normally sign on doesn’t go over 50k.
FB has recently started disregarding competing offers and just bumps your offer to the max limit they have. Then you either accept or decline.
Google in contrast can still go beyond the norm if you’ve perfectly matched a team and the hiring manager wants you badly.
Btw, the max for LinkedIn Senior (analog of E4 at FB) is 180/300/60, so FB can still be competitive if you consider other perks.