First of all, I’ve never talked to the guy, and even if I had, I’m not a psychiatrist. I can only speculate, so take this with a grain of salt.
I don’t think he’s motivated only by money. He plans to give 99% of his money away, and even if much of it will just go into a foundation that he’s also in charge of, this would seem to indicate that money isn’t a driving factor for him. Keep in mind that he’s worth about $50 billion, and could live a life of luxury for less than 1/1000th of that.
I suspect there are other metrics that matter to him: attention, respect from the general public (people will always remember him as the original billionaire twenty-something), and possibly a desire to undo the damage that the Social Network movie did to his perception.
I think he does genuinely care about Facebook’s mission of “connecting the world”, but I think he wants to do it on his own terms — meaning he wants to connect the world through Facebook — not through a bazaar of services that he doesn’t control.