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Within another year, the number of Facebook users tripled, and Zuckerberg's net worth hit $2 billion.
By 2010, after an investment by private equity firm Elevation Partners valued Facebook at $23 billion, Zuckerberg jumped to No. 35 on the Forbes 400 list, with a net worth of $6.9 billion.
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In 2011, Zuckerberg was worth an estimated $17.5 billion, but his fortunes took a turn for the worse in 2012, the same year Facebook went public.
Days after Facebook's IPO on May 18, 2012, shares tumbled on fears that the firm's shares had too lofty a valuation, shrinking Zuckerberg's net worth to just $9.4 billion.
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But by last year, as the number of Facebook mobile users and mobile ad revenue continued to grow, Zuckerberg's net worth grew more than it lost the year before.
He was worth $19 billion, according to the latest Forbes 400 list published last fall. And his fortune continues to grow. Zuckerberg's net worth is currently estimated at around $26.6 billion.
—By CNBC's Althea Chang.