Facebook's Zuckerberg tops list of America's 50 biggest donors

From Facebook's Mark Zuckerberg to a "secret millionaire," the country's 50 biggest donors were far more generous last year.

This crew donated a total of $7.7 billion in 2013, 4% more than in 2012, according to a new report from The Chronicle of Philanthropy. That brought the median donation to $86.1 million, up from $49.6 million in 2012.

Why so generous? The Chronicle credits last year's improving economy and booming stock market for the surge in donations.

Of all the donors, Facebook(FB, Fortune 500) co-founder and CEO Mark Zuckerberg and his wife Priscilla Chan were by far the most generous. The couple donated nearly $1 billion worth of Facebook stock, or 18 million shares, to the Silicon Valley Community Foundation. The massive gift made the organization, which provides grants to a variety of nonprofits, one of the biggest foundations in the country.

Zuckerberg and his wife (who are 29 and 28 years old, respectively) were also the youngest donors on The Chronicle's list, which had a median age of 72.

Related: More donations top $100 million

The second largest contribution was a $750 million bequest from George Mitchell, a billionaire known for pioneering the controversial practice of hydraulic fracturing or "fracking." He died in July at the age of 94.

Those funds will support the Cynthia and George Mitchell Foundation, which provides grants for clean energy, natural gas sustainability and other environmental projects. One of the foundation's main goals is to prevent environmental damage from the technology that Mitchell himself created, the Chronicle stated.

Other top donations included Nike(NKE, Fortune 500) co-founder Phil Knight's $500 million to the Oregon Health & Science University Foundation for research on the early detection of deadly cancers and former New York City Mayor Michael Bloomberg, who gave $452 million to a variety of arts, education, environment and public health nonprofits.

Meanwhile, few people knew how wealthy one of the top donors was until his death at the age of 98 last year. Dubbed a "secret millionaire" by media reports, Jack MacDonald was a thrifty former lawyer in the Seattle office of the U.S. Department of Veterans Affairs who left behind $139 million in 2013. MacDonald earned his millions by investing the proceeds from the sale of his father's meat company.

His will split his money among the Seattle Children's Hospital, University of Washington School of Law and the Salvation Army, which had no idea that the gift was coming.

"The Salvation Army didn't know at all," said Chronicle editor Stacy Palmer. "They only knew him as a donor who gave $20 at a time."

Related: America's biggest college endowments get even bigger

Philanthropists Bill and Melinda Gates and Warren Buffett were noticeably absent from the top 50 donor list, which is based on new financial commitments made in a given year. It does not track gifts that donors' foundations make using past commitments so as to not count the same money twice.

Bill and Melinda Gates and Warren Buffett's Giving Pledge campaign has been credited with inspiring philanthropy among other billionaires. The Chronicle notes that 19 of 2013's top 50 donors have signed the pledge.

The majority of the donations came from California ($2.8 billion) and New York ($1.6 billion) residents, while other large donations came from Texas, Oregon and Washington state residents.

Colleges and universities were the biggest winners, receiving $2.6 billion or roughly a third of all donations. Foundations received another $2.1 billion, while hospitals and medical centers, museums and library groups and human service groups were among the other recipients.