Warren Buffett is now worth $20 billion less than Mark Zuckerberg. Facebook's soaring stock isn't the only reason.

• Mark Zuckerberg passed Warren Buffett on the Bloomberg Billionaires Index last week, as the difference in their net worths ballooned to $20 billion.
• The 14% rise in Facebook stock this year has bolstered Zuckerberg's fortune, while the 23% decline in Berkshire stock has hammered Buffett's wealth.
• Another factor is that Buffett gave $15 billion to charity between 2014 and 2018, dwarfing the $1 billion given by Zuckerberg and his wife over the same period.
• However, Zuckerberg should catch up to Buffett eventually as both plan to give the vast majority of their fortunes to charity,
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Facebook CEO Mark Zuckerberg blew past Berkshire Hathaway CEO Warren Buffett in net worth last week. The $20 billion difference in their fortunes isn't just a product of Facebook stock outperforming Berkshire stock; it also reflects Buffett giving far more than Zuckerberg to charity.

Buffett's net worth has plunged by more than $20 billion this year to about $69 billion, while Zuckerberg's wealth has swelled by about $11 billion to north of $89 billion, according to the Bloomberg Billionaires Index.

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Their fortunes are closely tied to their respective companies' stock prices: Buffett owns about 16% of Berkshire, while Zuckerberg owns 13% of Facebook.

The roughly 23% slump in Berkshire shares since January has hammered Buffett's net worth, while the 14% surge in Facebook stock has boosted Zuckerberg's net worth over the same period.

However, the scale of their philanthropy is another big factor. Buffett gave nearly $15 billion to charity between 2014 and 2018, dwarfing the $1 billion given by Zuckerberg and his wife, Priscilla Chan, according to a list of America's 25 biggest givers compiled by Forbes and SHOOK Research.

There would be a much smaller gap in their net worths without those donations. Buffett would be worth about $84 billion, while Zuckerberg would be worth about $90 billion.

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It's worth noting their philanthropy gap is set to close in time, as both Buffett and Zuckerberg plan to give away almost all of their money.

"More than 99% of my wealth will go to philanthropy during my lifetime or at death," Buffett vowed as part of The Giving Pledge, an initiative he launched with Bill and Melinda Gates in 2006 to encourage billionaires to give away the majority of their fortunes.

"We will give 99% of our Facebook shares — currently about $45 billion — during our lives to advance this mission," Zuckerberg and Chan said in a public letter to their newborn daughter in 2015. The pair signed The Giving Pledge five years earlier, in 2010.

Here's a chart showing how Facebook rapidly caught up to Berkshire Hathaway in market capitalization, and eventually surpassed it, after going public in May 2012:

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