It's not just about money. If he has a majority share, and she ends up divorcing him and taking half of whatever she can get, it may destabilize the company. At that point, it's not just about Mark and his wife, there are jobs and people's well being's hanging in the balance.

It may not be about the money. If Prescilla has played a major role in making Facebook the $100B company then its OK. However there is no point splitting the company in half, because she was married to the founder. There is no need to demonize prenups here.