Mark Zuckerberg had a terrible, horrible, no good, very bad day.

The Facebook founder suffered a $3 billion loss in the value of his Facebook stock on Thursday, one of the largest single-day losses ever for an individual investor.

Facebook (FB) shares plunged 5.5% after the chief financial officer warned investors that the company expects revenue growth from advertising to "come down meaningfully."

Zuckerberg holds 418 million shares of Facebook stock -- still good for about $50.2 billion.

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Bloomberg estimates Zuckerberg's total net worth at $52.2 billion, ranking him as the fifth-richest man in the world, just behind Warren Buffett. And despite Thursday's plunge, Facebook shares are up nearly 15% for the year, lifting Zuckerberg's net worth by $6.4 billion.