Shares fell as much as 19.6 percent to $174.78, a decline that if sustained would wipe about $124 billion off the company's value - or almost four times the entire market capitalization of Twitter Inc. Investors also appear to have been alarmed by an alert from Facebook executives that revenue growth would slow as the company refocuses on improving user privacy.
Facebook has faced dozens of lawsuits over its handling of user data in a scandal also concerning the United Kingdom firm Cambridge Analytica.
Zuckerberg had been tied with Warren Buffett as the world's fourth-richest person, but the Berkshire Hathaway chairman's current $US83 billion ($A112 billion) fortune tops Zuckerberg's $US66 billion ($A89 billion), Forbes magazine said.
These quotes and average results from its second quarter caused Facebook's stocks to drop a whopping 19 per cent overnight.
Mr Buffett now ranks third among the world's billionaires, while Mr Zuckerberg is sixth.
The previous record drop was by Intel in September 2000, when it lost $91 billion in market capitalization.
Wehner added saying, "European ad revenue growth decelerated more quickly than other regions and was impacted primarily by reduced currency tailwinds and, to a lesser extent, the roll-out of GDPR".
But Facebook having fared well on the stock markets, most investors were not unduly bothered about Zuckerberg's leadership.
The S&P 500 technology index fell 2.0 per cent, the most among the major S&P sectors.
"Trade is bound to have an impact on the coming quarters if the tariff issue isn't resolved".
Shares in Alphabet closed up 0.7%, while those in Apple fell 0.3% and Netflix closed barely higher.