(JTA) — Facebook CEO Mark Zuckerberg and his wife are responsible for the largest on-record charitable donation in the United States in 2013.

For the second straight year, Zuckerberg and his wife, Priscilla Chan, gave away 18 million Facebook shares — a gift worth more than $970 million — to the Silicon Valley Community Foundation in December, the Associated Press reported Wednesday.

The donation was the largest charitable gift on public record in 2013, according to the Chronicle of Philanthropy, and marked the first time that donors under the age of 30 were responsible for the year’s largest donation.

Zuckerberg and Chan’s 2013 donation was equal in shares to the gift the couple gave the same foundation in December 2012. That gift was valued at just under $500 million at the time, but the success of Facebook’s stock over the past 12 months more than doubled the value of the 2013 donation, the news website Mashable reported.

The Silicon Valley Community Foundation serves as a gatekeeper to a number of charitable causes. In 2013, the foundation awarded more than 10,000 grants to over 29 different countries, according to its website.

Nike Chairman Phil Knight and his wife Penelope were responsible for the second-largest donation on public record in 2013: a $500 million pledge to the Oregon Health & Science University Foundation for cancer research.