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The 26-year-old is one of 16 billionaires new to the pledge, which now totals more than 50 donors. New names include AOL co-founder Steve Case, investor Carl Icahn and former junk-bond king Michael Milken.

They join existing pledges made by wealthy individuals and families including Oracle Corp. founder Larry Ellison, film director George Lucas and New York Mayor Michael Bloomberg.

The Giving Pledge is an effort organized by software mogul Bill Gates and investor Warren Buffett to persuade the world's rich to boost their giving.

"I view this as a call to others who might in their thirties or forties use some of their creativity to get involved in philanthropy earlier in life," Mr. Milken, 64, said of the pledge.

Those pledging are part of a broader shift in philanthropy, in which successful business people—often entrepreneurs—are giving more of their money to charity far earlier than their predecessors. It was a trend that was helped along by Mr. Gates, who started his foundation while still leading Microsoft Corp.

Mr. Case, 52, and his wife Jean Case, 50, said they signed the pledge because they hoped it would help philanthropists learn from each other. "It is less about what size of a check that you write and more about the outcome," Mr. Case said.

Ms. Case said Internet entrepreneurs have a unique interest in philanthropy. "The folks that helped bring AOL to life were out to change the world," she said. "It seems a natural thing that as they look at the role they want to play, they are giving back in big ways."

Messrs. Icahn and Zuckerberg weren't available to comment. In a video prepared by the Giving Pledge, Mr. Zuckerberg said, "There's so much that needs to be done, it would be better to start now."

The Giving Pledge was born in part from a dislike by Mr. Buffett for dynastic wealth. Mr. Buffett over the years has schooled Mr. Gates on philanthropy, giving him a copy of "The Gospel of Wealth," in which steel tycoon Andrew Carnegie argued that fortunes were often wasted by heirs and thus should be put to charitable use.

Starting last year, Mr. Gates, his wife Melinda Gates, Mr. Buffett and other wealthy individuals hosted a series of dinners for billionaires to discuss setting up the pledge. That led to an announcement in June of the pledge and its earliest signers.

Since then, Mr. Gates, Ms. Gates and Mr. Buffett have been calling on billionaires to get their commitment. The pledge doesn't ask for specific donations, nor does it track giving; rather, it asks that a pledge maker commit to giving away the majority of their wealth.

That proposition at times has been a tough sell, the pledge founders said. "People are super nice to us, but there is a certain awkwardness because it's a big decision," Mr. Gates said in an interview this week. "Sometimes the wife and the husband have never really talked through their priorities on the charity stuff."

Mr. Zuckerberg, who founded Facebook in his Harvard University dorm before dropping out of college and working on the business full time in California, is one of the world's youngest billionaires, worth an estimated $6.9 billion, according to Forbes. Yet since his wealth is from his ownership stake a company that has yet to list on the stock market, much of that wealth is theoretical at this point.

Dustin Moskovitz, a co-founder of Facebook and former Harvard roommate of Mr. Zuckerberg's, has also signed the pledge.

Mr. Zuckerberg emerged as a major donor this year. In September, he made his first major charitable gift, promising on "The Oprah Winfrey Show" to give up to $100 million to public schools in Newark.

Many of the pledge signors had already planned to disburse their wealth and most are already involved in philanthropy. It's unclear if the Giving Pledge has encouraged more giving.

Overall, philanthropic giving has been hit hard by the weak economy. Donations in the U.S. fell 3.6% to $303.75 billion last year, down from $315 billion in 2008, according to Giving USA. In 2008, they were down 2%, Giving USA Foundation.

Investor and new addition to the pledge, Nicolas Berggruen, 48, said he decided to give away his fortune while he was alive so he could personally take responsibility for how his money is put to use.

"Wealth is an advantage, but it also is frankly a responsibility," he said in an interview.

After the initial Giving Pledge list came out, some critics decried it as a public-relations stunt, or the product of tax-breaks that are hurting the government's ability to offer critical services.

"The state has limits in to what it can and cannot do," said Mr. Berggruen. "Private enterprise can be faster and less bureaucratic than the state."

Write to Robert A. Guth at rob.guth@wsj.com and Geoffrey A. Fowler at geoffrey.fowler@wsj.com