Mexican telecom mogul Carlos Slim Helu tops Forbes\' annual list of billionaires for the third consecutive year with a personal fortune of $69 billion US. Microsoft co-founder Bill Gates is second with a fortune of $61 billion, while investor Warren Buffett is third at $44 billion. Slim\'s personal net worth slipped by $5 billion over the past year as stock prices of companies he owns lagged. He made much of his fortune through telecommunications companies, and has more recently expanded into commodities, retail and financial enterprises. Former world\'s-richest-man Gates closed the gap over the past year, adding $5 billion as Microsoft shares hit a10-year high. Gates\' fortune would be much higher if it weren\'t for billions of dollars he\'s given to the Bill and Melinda Gates Foundation. Both Gates and Buffett have pledged to donate most of their money to the foundation upon their deaths, and have lined-up dozens of other billionaires to follow suit. Facebook co-founder Mark Zuckerberg continues to rise up the ranks of world billionaires. He is now estimated to be worth $17.5 billion, 35th in the world. His fortune rose $4 billion from last year and could rise much more when the company goes public later this year. In 2011, Zuckerberg ranked 52nd in the world with an estimated worth of $13.5 billion. That was up from the 212th and $4 billion in 2010. Other Facebook co-founders Dustin Moskovitz and Eduardo Saverin also rank among the world\'s billionaires, with fortunes of $3.5 billion and $2 billion respectively. The 27-year old Moskovitz is also the world\'s youngest billionaire, just a few months younger than Zuckerberg. The oldest is 101-year-old Swiss software pioneer Walter Haefner. The world\'s richest woman remains Lillianne Bettancourt, daughter of L\'Oreal cosmetics founder Eugene Schuller. The 89-year old is worth $24 billion. Canada\'s wealthiest man continues to be David Thomson. The media titan and part owner of the Winnipeg Jets is worth $17.5 billion, down $5.5 billion from a year earlier. Thomson is among 25 Canadian billionaires, including Cirque du Soleil founder Guy Laliberté and Lululemon founder Chip Wilson. According to Forbes, there are 1,226 billionaires in the world, an all-time high. More than a third of them are from the United States. The combined net worth of all the world\'s billionaires is $4.6 trillion, up 2 per cent from a year ago, while the average fortune is $3.7 billion. Over a third of all the world\'s billionaires saw their net worth drop last year. India’s Lakshmi Mittal was the biggest \"loser\" from the previous list. The owner of the world\'s largest steel company saw his fortune fall by $10.4 billion to $20.7 billion, knocking him out of the top 10 for the first time since 2004.