Forbes magazine is due to release its annual list of the world's billionaires around midday on Wednesday, with Mexico's Carlos Slim poised to retain the top spot and attention focused on how far up the list Facebook's IPO has catapulted Mark Zuckerberg.

The list is due out online sometime after noon EST (1700 GMT), ranking each of the world's more than 1,000 billionaires in the magazine's 25th annual issue on the world's wealthiest people.

Slim made much of his fortune as a telecommunications magnate who has expanded into retail, finance, commodities and energy.

In recent years he has taken larger stakes abroad, owning parts of department store operator Saks Inc, publisher New York Times Co and money manager BlackRock Inc. In June he sold his stake in oil services company Bronco Drilling for a tidy profit.

Last year he expanded his Forbes lead over Bill Gates and Warren Buffett, in part because those two men, especially Gates, have given away much of their fortunes to charity.

While Slim has become involved in combating poverty, illiteracy and poor healthcare in Latin America and in promoting sports projects for the poor, he has never voiced plans to give away large chunks of his wealth to charity.

Zuckerberg and other Facebook billionaires could move up the list this year after the company detailed plans on February 1 for an initial public offering valuing the company at a projected $100 billion.

On paper, Zuckerberg's net worth should take another leap considering his 28.4 percent stake in the company.

For the 2011 Forbes list, the Facebook cofounder and chief executive ranked No. 52, with an estimated worth of $13.5 billion. That was up from the 212th position and $4 billion in 2010.

(Reporting By Daniel Trotta; editing by Matthew Lewis)

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