Stupid Question?
"All of which leaves us wondering if Zuck will change his mind about the pledge if his real world fortunes were to plummet?"
IMHO that is a stupid question - I imagine that almost ANYONE experiencing a serious "change in fortune" would modify their charitable commitments. Or to put it another way - kind of hard to give away what you don't have.
Based on half of what I've read, Zuck is probably a wee bit of a schmuck; but if he's willing to try and give healthy contributions to charity then perhaps it would be reasonable to lay off?