Microsoft co-founder Bill Gates has topped the Fortune 400 list as the wealthiest American for the 20th year in a row. His total net worth of $72 billion kept him well ahead of Warren Buffett's second place finish of $58.5 billion. Gates saw his fortune increase by $6 billion over the past year while Buffett's wealth ballooned by $12.5 billion during the same period.

Oracle CEO Larry Ellison's $41 billion nest egg remained flat over the year and was good for a third place finish, cementing the technology industry as the second-largest generator of billionaires. Nearly 50 of the people on the top 400 list are in the tech industry which accounts for a 12 percent share of the overall money pool.

Further down the list is Amazon's Jeff Bezos (12th place) at $27.2 billion. Following in 13th and 14th place are Google co-founders Larry Page and Sergey Brin. Collectively, the Google duo are worth $49.3 billion. Facebook founder Mark Zuckberger is one of the youngest billionaires on the list. His estimated wealth of $19 billion is up thanks to the company's recent turnaround on the stock market following a year of dismal results.

Interestingly enough, outgoing Microsoft CEO Steve Ballmer placed 21st on the list at $18 billion. Last month, Ballmer announced plans to retire within the next 12 months. He has held the title of Microsoft CEO for nearly 13 years following Gates' departure in January 2000.