Facebook CEO Mark Zuckerberg returned from paternity leave this week, NBC News reports. As a welcome-back gift, he officially became the world's sixth-richest person on Thursday. According to Bloomberg, his net worth increased by $6 billion on the stock market Thursday, lifting him past the Koch brothers. The 31-year-old is now worth $47.5 billion. He's currently behind only Bill Gates ($77.9 billion), Amancio Ortega ($70.2 billion), Warren Buffett ($59.1 billion), Jeff Bezos ($51.9 billion), and Carlos Slim ($48.6 billion). But those five have all lost money this year to the tune of a combined $19 billion.
The news prompted CNN to raise the possibility that Zuckerberg "might be the best CEO in America." Facebook is less than 12 years old but is worth more than $300 billion. The $1 billion it paid for Instagram "has turned out to be an amazing bargain." And it's seemingly mastered mobile, with 80% of its ad revenue coming from that sector. According to Bloomberg, Facebook announced Wednesday that sales were up 52% in the fourth quarter and it had more than doubled its net income over the previous year. NBC points out Zuckerberg has promised to give most of his fortune away. (Read more Mark Zuckerberg stories.)