It is always about the money. The more money you have the more money you need to manage the money you already have. So it is always about the money. The Zuck was confident his website was worth way more than a billion. that's why he didn't sell.
If it was about money, he wouldn't be giving away most of his company while trying to retain a controlling stake. The fact that he is giving away his money suggests that he's more interested in being a successful business mogul than he is about being "FU rich."