Facebook's Mark Zuckerberg and his wife were the second-biggest charitable donors in the United States last year, appearing behind only billionaire Warren Buffett on a list of the nation's most generous philanthropists.Zuckerberg, along with wife Priscilla Chan, gave roughly half a billion dollars ($498.8 million) to the Silicon Valley Community Foundation, according to The Chronicle of Philanthropy.The group released its annual Philanthropy 50 over the weekend.The Facebook founder and CEO was one of three tech-industry titans in the list's top five.Microsoft co-founder Paul Allen was fourth, with $309 million in contributions, mostly to the Allen Institute for Brain Science.And Google co-founder Sergey Brin and wife Anne Wojcicki, co-founder of genetic-testing company 23andME, were fifth, putting $223 million into their Brin Wojcicki Foundation.Zuckerberg, whose net worth is probably somewhere around $12 billion despite a rocky start for Facebook's stock, made his first big splash in the philanthropy space in 2010, when he pledged $100 million to public schools in Newark, New Jersey.The Silicon Valley Community Foundation issues grants for a host of causes in the San Francisco area. In 2012, its charitable causes ranged from programs to teach immigrants English, to groups providing food and shelter to the needy, to funds for victims of the California wildfires.Zuckerberg donated 18 million Facebook shares to the foundation in December."Together, we will look for areas in education and health to focus on next. I'm hopeful we'll be able to have as positive an impact in our next set of projects," Zuckerberg wrote on his Facebook page at the time.Allen's institute is devoted to research on how the human brain works, particularly in regard to health and disease. It is a nonprofit medical research organization launched in 2003 and which Allen compares, in scope, to the Large Hadron Collider and the Human Genome Project.Brin and Wojcicki's foundation donates to a number of causes which, in 2012, included women's and environmental issues and a foundation devoted to curbing poverty. The couple also donated nearly $33 million last year to the Michael J. Fox Foundation for Parkinson's Research.Zuckerberg, 27, and Brin, 39, contributed to a trend noted by the chronicle -- and likely spurred by the emergence of the youth-oriented tech field. Three of the year's top five donors were under 40. The other, No. 3 on the list, was hedge-fund creator John Arnold, who is 38, and his wife, Laura.