Once the IPO happens and you pay your employees $4.5 million, their $100-$200k salary doesn't seem worth much anymore. What actually would have happened to Facebook, if they did this? Is it possible that the company collapses, while too many employees quit so they can do their own thing, or retire? Would they have had to double or triple everyone's salaries to keep them on board?
I could see it being a disaster. It could also significantly de-value a company's IPO (or sale) because the investors would see this as a massive risk. At least until it's been tested with a company that IPOs.