Mark Zuckerberg and his wife, Priscilla Chan, were the most generous American philanthropists in 2013, with a donation of 18 million shares of Facebook stock, valued at more than $970 million, to a Silicon Valley nonprofit in December. The Chronicle of Philanthropy reported Monday that Zuckerberg's donation was the largest charitable gift on the public record in 2013 and put the young couple at the top of the magazine's annual list of 50 most generous Americans in 2013. The top 50 contributors made donations last year totaling $7.7 billion, plus pledges of $2.9 billion. The Chronicle's editor says the most significant fact from the list was the amount of money coming from living donors, which totaled about the same amount as the two previous years combined. "It's a sure sign that the economy is getting better and people are getting a lot less cautious," said Stacy Palmer, Chronicle editor. Some of the nation's biggest givers do 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.