Jeff Bezos is the world's newest US$100 billion ($145b) mogul.

The Amazon.com founder's fortune is up US$2.4b to US$100.3b as the online retailer's shares jumped more than 2 per cent on optimism for Black Friday sales.

Online purchases for the day are up 18.4 per cent over last year, according to data from Adobe Analytics, and investors are betting the company will take an outsized share of online spending over the gifting season.

The US$100b milestone makes Bezos, 53, the first billionaire to build a 12-figure net worth since 1999, when Microsoft co-founder Bill Gates hit the mark.

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Bezos's fortune rose US$32.6b this year through Thursday, the largest increase of anyone on the Bloomberg Billionaires Index, a daily ranking of the world's 500 richest people. Amazon have climbed 5 per cent this week alone.

As Bezos' wealth flirts with new heights, there's likely to be more questions about what he intends to do with it. Unlike Gates, who was the world's richest person until Bezos passed him in October, or US investor Warren Buffett, the world's third-richest person with US$78.9b, Bezos has given relatively little of his fortune to charity.

Bezos is only just starting to focus on philanthropy, and in June tweeted a request for ideas on how to help people.

Since 2002, Bezos has given away Amazon shares worth about US$500m at current prices, according to a Bloomberg analysis of Form 4 filings. The billionaire said in April that he sells US$1b of Amazon stock every year to fund his space business Blue Origin.

Gates, 62, who has a net worth of US$86.8b according to the Bloomberg index, would be worth more than US$150b if he hadn't given away almost 700 million Microsoft shares and US$2.9b of cash and other assets to charity, according to an analysis of his publicly disclosed giving since 1996.

The index numbers are based on the close of trading in New York Wednesday. Bezos's fortune and Amazon share data reflect Friday's closing trading prices in New York.

- Bloomberg