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(CNN) - America's wealthiest philanthropists went big last year, donating more than $3.4 billion to charity, according to a new tally by the Chronicle of Philanthropy.
Number 3: Mayor Michael Bloomberg
Pledged $350 million to Johns Hopkins University
"He has given a lot of money to that institution, more than a billion dollars," said Stacy Palmer, editor for the Chronicle of Philanthropy. "He gives to many other causes as well, and now that he is leaving the mayor job, we expect him to be more full-time philanthropist."
Bloomberg pledged to give away all of his fortune to charity during his lifetime.
"He has a lot of work to do, because there are many more billions to give away," said Palmer.
Number 2: Nike chairman Phil Knight, and his wife
Pledged $500 million to Oregon Health and Science University Foundation
This gift comes with a hitch.
"The university has two years to raise another $500 million from other donors, or they don't get the money," said Palmer. "They have to match the money."
But even if they don't meet the challenge, "I have a feeling he might decide to give it to them anyway," said Palmer. "He really wants to encourage other people to give, and the best way to do that is to say, 'Here, I am giving this generous gift, why don't you join me.'"
Number 1: Mark Zuckerberg and his wife Priscilla Chan
Pledged $992.2 million to the Silicon Valley Community Foundation
Zuckerberg is just 29.
"It is remarkable. We have never seen somebody top the list who is under 30," said Palmer.
"It is an incredible gift to see somebody give this much. And this is his second big gift to the foundation. He gave a half billion last year," said Palmer.
The charitable trends of 2013 were donations to colleges and universities – 12 of the 15 donations went to colleges, but went to a range of causes, some gifted alma maters for buildings, others to advance stem cell or cancer research.
"One reason is that colleges seem like trustworthy organizations. And if you're going to give many millions of dollars, you may want to give to an organization you trust," said Palmer. "Also colleges are good at asking for the money."
Warren Buffett topped the Chronicle of Philanthropy's list in 2012, but didn't crack the top ten for 2013. That's because the list focuses on new gifts only.
"I suspect he did give money, but he just didn't make the list by going public with it," said Palmer.
And in case you're wondering, there is a bit of philanthropy competition between multi-billionaires.
"We've heard from one very rich person who already was upset that he wasn't on the list," said Palmer. "It does matter to them, they do like to be seen for that ... they want to be known as doing good."
If you're looking for a way to make your own gift to those in need, check out CNN.com/impact.