* Record 1,266 billionaires average $3.7 bln each
* U.S. leads all countries with 425 billionaires
* With 96 billionaires, Russia No. 2 country
* Mexico's Salinas Pliego adds $9.2 bln to his wealth
* India's Mittal loses $10 bln as steel shares tumble
NEW YORK, March 7 (Reuters) - Mexican tycoon Carlos Slim retained his position atop Forbes magazine's annual list of the world's billionaires on Wednesday with an estimated worth of $69 billion, while his Mexican rival Ricardo Salinas Pliego enjoyed the largest increase in wealth.
Microsoft Corp cofounder Bill Gates ranked second at $61 billion and Warren Buffett third at $44 billion. France's Bernard Arnault, the richest person in Europe, repeated at No. 4. They were the same top four as last year.
The youngest self-made woman at age 41 was Spanx founder Sara Blakely, whose net worth of $1 billion from her business of making slimming undergarments affirmed there was profit in vanity. Blakely, ranking at No. 1,153, was among 104 women on the list.
Fifty-eight countries were represented on the list of a record 1,226 billionaires whose average fortune was $3.7 billion. Some 128 billionaires were new to the Forbes rankings this year, including Colombia's Alejandro Santo Domingo, 35, the richest new billionaire in the world ($9.5 billion) and the face of Santo Domingo Group after last year's death of his father.
In addition to the usual aging plutocrats and heirs, Facebook founder and CEO Mark Zuckerberg, 27, ranked 35th with a net worth of $17.5 billion. The social media heavyweight announced plans last month for an IPO that would value Facebook at a projected $100 billion.
Zuckerberg jumped 17 spots and $4 billion while the combined value of seven Facebook billionaires was $28.7 billion, including the $3.5 billion belonging to Dustin Moskovitz, 27, Zuckerberg's former Harvard roommate who was listed as the youngest self-made man.
Notable drop-offs included "Harry Potter" author J.K. Rowling, whose wealth fell below $1 billion due in part to Britain's high tax rates and an estimated $150 million in charitable giving.
Also falling off was Jim Balsillie, the former co-CEO and co-chairman of Canada's Research In Motion, the troubled maker of BlackBerry.
RUSSIA THE NEW NO. 2
The United States led the rankings with 425 billionaires, seemingly unfazed by the burgeoning "Occupy" movement which sought to shine a light on income inequality. Russia overtook mainland China as the country with the second-most billionaires, by a margin of 96 to 95. New to the list was Morocco, which had three billionaires.
Among cities, Moscow led with 78 billionaires followed by New York at 58 and London at 39. The full list can be seen at www.forbes.com/billionaires/
Slim, 72, and his family have taken the top spot for three years in a row, climbing up the list for years based largely on assets from his telecommunications empire now led by America Movil .
Slim made much of his money 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.
His fellow Mexican, Salinas Pliego, was the world's biggest gainer in dollar terms, adding $9.2 billion to his net worth and ranking at No. 37 on the list. Like Slim, he has benefited from the clubby nature of Mexico's business elite, building an empire that includes the country's No. 2 broadcaster, TV Azteca , and retail conglomerate Grupo Elektra .
In contrast to Salinas Pliego, Forbes listed India's Lakshmi Mittal as the biggest loser, saying Mittal lost $10.4 billion, falling to a net worth of $20.7 billion and out of the world's top 10 for the first time since 2004. The drop came after shares in his ArcelorMittal, the world's largest steelmaker, fell due to surging costs and soft demand in Europe.
Microsoft's Gates narrowed the gap with Slim this year after the Mexican tycoon's estimated net worth fell from $74 billion to $69 billion. Forbes estimated Gates' net worth rose from $56 billion to $61 billion.
Forbes now has a competitor in the billionaire-listing business: the Bloomberg Billionaires Index, which debuted on Monday with a Top 20 list that is updated each day. Bloomberg hired away Matthew Miller, a former editor of the Forbes list, more than a year ago to head up compilation of the list.
Forbes will launch its Billionaire Real-Time Ticker this month with 50 of the top billionaires updated in real time.
The Bloomberg list will exclude New York City Mayor Michael Bloomberg, the founder and majority owner of Bloomberg LP, owing to a Bloomberg News policy not to cover Bloomberg LP, a major competitor to Thomson Reuters.
Michael Bloomberg is ranked at 20 in the Forbes list at $22 billion.
Laurene Powell Jobs, the widow of Apple co-founder and CEO Steve Jobs, who passed away in October 2011, entered the ranking at 100, and is worth $9 billion.