WASHINGTON (AFP) - The surging price of Microsoft shares returned US tech tycoon Bill Gates back to the top of Forbes s world s billionaires list, with his $76 billion beating out Mexico s Carlos Slim s $72 billion.

The annual list, released Monday, counted 1,645 men and women as billionaires, with an average wealth of $4.5 billion and a collective wealth of $6.4 trillion, up $1 trillion from a year ago.

Gates, co-founder of the US software firm, showed his staying power at the top -- the world s richest man for 15 of the past 20 years, according to Forbes -- despite spending recent years giving away large sums of money to global health and anti-poverty programs.

Gates owns about 4.4 percent of Microsoft, making up less than 20 percent of his total fortune.

But the company s share price has risen 25 percent over the past year, and, along with gains in other assets, he has added $7 billion to his fortune since a year ago, according to Forbes.

Slim, with a hand in everything from telecommunications (America Movil) to mining, finance and industry (Grupo Carso), to retailing and real estate across the Americas region, was worth $1 billion less than a year ago, hit in part by sagging markets in South America.

In third was Spain s Amancio Ortega, whose pockets have filled with profits from fashion: his hugely successful Inditex garment empire, parent of popular chains Zara, Pull & Bear, and Bershka. More recently, Ortega heavily invested in real estate in Europe and the United States; his worth was put at $64 billion.

A familiar cast of mega-wealthy filled out the rest of the top ten: US investment guru Warren Buffett ($58.2 billion); software group Oracle s founder and chief executive Larry Ellison ($48 billion); US industrialists and brothers Charles and David Koch (each with $40 billion); Las Vegas casino king Sheldon Adelson ($38 billion); Walmart heiress Christy Walton ($36.7 billion) and her brother Jim Walton ($34.7 billion).

Together the $507 billion held by the top ten is larger than the entire size of the economy of Norway, or Belgium or Poland in 2012.

- 31 billionaires under 40 -

US billionaires dominated the list, with 492, followed by 152 from China and 11 from Russia.

Forbes said two-thirds of them made their own fortunes, while 13 percent inherited. The rest took fortunes they received and built them up to the $1 billion-plus level.

The biggest climb on the list in the year came from Facebook s Mark Zuckerberg. The public sale of the company s stock more than doubled his net worth to $28.5 billion, leaving him at number 21, just behind Hong Kong s venerable tycoon Li Kashing and just ahead of Italian chocolatier Michele Ferrero.

Zuckerberg, 29, led a group of 31 billionaires under 40 years old. Youngest, at age 24, was Perenna Kei -- also known as Ji Peili -- the 85 percent shareholder of real estate company Logan Property Holdings, which went public in Hong Kong in December. Her father Ji Haipeng is chairman and chief executive of the company.

Saudi investor Prince Alwaleed bin Talal, a nephew of the oil giant s king, was likely to remain unhappy with Forbes after blasting it last year for allegedly understating his wealth.

Forbes put his fortune this year at just $20.4 billion -- enough for a ranking of number 30 -- compared to $20 billion a year ago.

Defending itself last year, Forbes said the prince "systematically exaggerates" his wealth.

Last week Alwaleed sent out a press release noting that the Bloomberg Billionaires Index, a rival of Forbes, put his fortune at $30.5 billion.

In the statement, he assailed Forbes for "use of incorrect data" and said its approach "seemed designed to disparage Middle Eastern financial institutions such as the Saudi stock exchange."