International charity Oxfam's report Monday saying that the richest 62 people hold as much wealth as half the world's population, as well as a pledge in December by Facebook founder Mark Zuckerberg and his wife to donate 99 per cent of their company shares, have put the spotlight on how billionaires give back.

Here's a look at the documented philanthropic activities of a few of the richest people on the Forbes 2015 billionaire ranking list:

Bill Gates 

windows_xp_620x412

Bill Gates was ranked the world's richest man by the Forbes 2015 World's Billionaires list. (Anthony P/Reuters)

According to Forbes, Microsoft's Bill Gates leads the 2015 billionaires list with a net worth of $79.2 billion US ($115 billion Cdn). His philanthropic work through the Bill and Melinda Gates Foundation may also be among the most widely known. The foundation largely funds global development and health projects, including polio immunization and prevention of HIV/AIDS, malaria and other diseases.  

Carlos Slim 

Carlos Slim Helu

Mexican telecom mogul Carlos Slim, ranked second on the 2015 Forbes list of richest billionaires, was first on the list from 2010 through 2013. (Jeremy Piper/Associated Press)

​With a listed net worth of $77.1 billion US ($112 billion Cdn), Mexican telecom giant Carlos Slim nabs second place on the Forbes list. However, his approach to philanthropy appears to differ from many of his counterparts.

Slim has not signed up for The Giving Pledge started by Bill and Melinda Gates and business icon Warren Buffett, in which people commit to giving more than half of their wealth to charitable causes. Slim has said that the key to eliminating poverty is not charitable organizations, but giving people jobs. 

"Foundations do not solve poverty," he said in an interview with the Guardian published in December. "Employment requires that companies invest … we need to create companies."

Having said that, Slim does have a foundation in his name and made the Forbes "World's Biggest Givers" list in 2011. According to Slim's website, his foundation's mission is to create non-profit projects in "education, health, justice and personal and community development by contributing human and financial resources to equip Mexican society with the necessary tools to succeed professionally and socially." 

Mark Zuckerberg

Facebook Scheme Arrest

Facebook CEO Mark Zuckerberg ranks No. 16 on the Forbes 2015 billionaires list with a net worth of $33.4 billion US ($48 billion Cdn). (Paul Sakuma/Associated Press)

Ranking No. 16 on the Forbes 2015 list of billionaires, Facebook founder Mark Zuckerberg and his wife, Priscilla Chan, drew both praise and cynicism when they published a letter to their newborn daughter on Facebook in December pledging to contribute most of their company shares — worth about $45 billion US —  to improve the world for the next generation. 

In that letter, the couple announced the beginning of the Chan Zuckerberg Initiative, saying its priorities would include curing disease, providing internet access to all and "connecting people and building strong communities."

However, their philanthropic approach was questioned after some experts pointed out that their new initiative was not a charity or foundation, but a limited liability company (LLC) that is not subject to the same kinds of rules and public transparency.  

Lawrence Ellison​

LAWRENCE ELLISON CEO OF ORACLE TESTIFIES BEFORE SENATE JUDICIARY COMMITTEE.

Oracle Corporation founder Lawrence Ellison is the fifth richest billionaire in the world, according to the Forbes 2015 ranking. (Reuters)

Some billionaires, like Oracle founder Lawrence Ellison, focus their philanthropic support on specific causes. According to its website, the Lawrence Ellison Foundation awards grants for biomedical research on aging.

"By the year 2025, 1.2 billion people will be 60 or older," the website says. "Improvements in health care and disease prevention have the potential to create economic benefit to, and to dramatically improve the quality of life of, millions of individuals." Listed research areas include genes and aging, stem cells, as well as the effect of aging on various parts of the body. 

Ellison ranks fifth on the Forbes 2015 billionaires list. Like Gates and Zuckerberg, he has committed to The Giving Pledge.  

Charles and David Koch

Charles and David Koch

U.S. billionaire brothers Charles Koch, left, and brother David Koch are tied for sixth place on the Forbes 2015 wealth ranking, with a net worth of $42.9 billion US ($62 billion Cdn). (Mike Burley/Topeka Capital Journal/Associated Press and Brendan McDermid/Reuters)

Charles and David Koch, sons of Koch Industries founder Fred Koch, are tied for sixth place on the Forbes list. Neither of them is listed on The Giving Pledge website. Each brother has a foundation in his name. 

The Charles Koch Foundation funds non-profit projects, colleges and universities and professional education programs to "advance an understanding of how free societies improve the well-being of people around the world," according to the organization's website. Some of the recent grants listed include research into alternatives to incarceration and a scholarship examining the U.S. approach to foreign policy. 

His brother, David Koch, is also a "passionate believer in free societies," according to the David H. Koch Charitable Foundation website. A prostate cancer survivor, Koch's foundation cites large gifts to medical and cancer research, including $100 million US to a cancer research centre bearing his name at the Massachusetts Institute of Technology and $150 million US to the Memorial Sloan-Kettering Cancer Centre.

David Koch's foundation has also donated $35 million US to renovate the dinosaur hall at the Smithsonian's National Museum of Natural History; $65 million US to an outdoor plaza at the Metropolitan Museum of Art; and $100 million US to renovate the State Theater of New York at the Lincoln Centre. All three facilities were named after Koch. 

Li Ka-shing

Li Ka-shing

Hong Kong tycoon Li Ka-shing has donated millions of dollars to Canadian universities and St. Michael's Hospital in Toronto. (Kin Cheung/Associated Press)

According to Forbes, businessman Li Ka-shing is the richest person in Hong Kong and the 17th wealthiest on the 2015 billionaires list. His foundation funds health and education projects worldwide, including Canada.

The Li Ka Shing Foundation website lists five Canadian beneficiaries: the University of Toronto/St. Michael's Hospital Li Ka Shing Knowledge Institute; the University of Alberta Institute of Virology; the McGill University Student and Faculty Exchange Programs; a University of Calgary Endowed Professorship; and the University of Manitoba Academic Exchange Program. 

With files from Alexsandra Sagan and from Reuters