I'd say it's a big deal because $850M donated to a worthy cause (e.g., GiveWell) could have made a big difference in the world. I've suffered far more for far less.
If you're going to call him out for not staying at FB long enough to get the $850M to donate it somewhere, why not also call him out for not donating most of his $3B fortune? Surely he could scrape by on even a paltry $1B.
What's the difference from telling someone to stick it out long enough to vest more stock versus telling him to just donate some portion of his already vast fortune?
It would be good if he did either or both of those things. The original question was why giving up $850,000,000 is a big deal when you're already super-rich. The answer is that, even if it won't affect your living standards, it's still worth $850,000,000 to the people you could use it to help.
Giving up $850M when you already have $2B doesn't seem like much of a sacrifice, and is unlikely to change his life at all.
Giving up $850K when you have $2M would be a bigger sacrifice than giving up $850M when you already have $2B.