MacKenzie Bezos, who came away as the fourth-richest woman in the world following her divorce from Amazon founder and CEO Jeff Bezos earlier this year, comes in at #15 on the Forbes 400 list with a total net worth of $36.1 billion . She pledged in May to give away half of her wealth via The Giving Pledge, an organization created by Bill and Melinda Gates and Warren Buffett that invites billionaires to publicly dedicate more than half of their wealth to philanthropy.