Unless he was in a very unusual position of leading some kind of research project related to Facebook's core business, there will be close to no damage. It's a huge corporation, not a bootstrapped startup, everyone is highly replaceable.
Re:I'm resigning from my job at Facebook
by testbot123 on Thursday June 04, 2020 @11:46AM
(#23418157)
Luckily, he made a very public post about it which is currently being discussed in one of the most influential tech news aggregators on the internet. If others are inspired and also leave the company, it will make an impact.
Of course at Facebook's scale it's a small number, but given the damage to their reputation, they will have to take note once this crosses critical mass.