Facebook CEO Mark Zuckerberg and his wife Priscilla Chan, a pediatrician, topped the list of the most generous American philanthropists in 2013 with a donation of 18 million shares of Facebook stock that are now worth more than $1 billion. According to the Chronicle of Philanthropy, it was the largest charitable gift on the public record in 2013. On Friday, they announced a $120 million gift to the San Francisco Bay Area public school system.

Some of the nation’s biggest givers, including Microsoft co-founder Bill Gates and his wife, Melinda, did not appear on the 2013 list, not because they stopped being generous, but because their donations in 2013 were counted as pledges in previous years.