I read that most of the crazy comp Zuck is offering is in stock. So in a way, going to the place where they have lots of stock reflects their belief about where AGI is going to happen first.
Facebook is already public, so they can sell the day it vests and get it in cold hard cash in their bank account. If Facebook weren't public it would be a more interesting point as they couldn't liquidate immediately, but they can, so I wouldn't read anything into that.
But maybe the salary is also higher?
Comp is comp, no matter how it comes (though the details can vary in important ways).
I know people who've taking quite good comp from startups to do things that would require fundamental laws of physics to be invalidated; they took the money and devised experiments that would show the law to be wrong.