Facebook could potentially lose $151 billion in market capitalization when trading opens on the New York Stock Exchange on Thursday, 26 July.
He accused Facebook - particularly Chief Executive Officer Mark Zuckerberg and Chief Financial Officer David Wehner - of nurturing misleading statements and failing to disclose its slowing revenue growth.
Kacouris said the marketplace was "shocked" when "the truth" began to emerge on Wednesday from the Menlo Park, California-based company.
A Facebook spokesman declined to comment.
Earlier this year, Facebook was embroiled in the Cambridge Analytica scandal, and took big steps to make the platform more personal and secure immediately after.
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Facebook had the largest-ever loss of value in one day for a U.S. traded company. This wiped as much as $150 billion from the company's market value.
The more than $15 billion in net worth that Zuckerberg lost on Thursday is roughly equal to the wealth of the world's 81st-richest person, now Japanese businessman Takemitsu Takizaki, according to Forbes real-time data.
This allowed Buffer to rank third among the world's billionaires, while Zuckerberg ends up in the sixth spot.
Shares in Alphabet closed up 0.7%, while those in Apple fell 0.3% and Netflix closed barely higher.
Founder Mark Zuckerberg, who has a 13 per cent stake in Facebook, saw his fortune dropped by more than $US12 billion ($A16 billion) in less than 24 hours, to around $74 billion ($A100 billion). The company was worth over 630 billion dollars the previous day.
As expected, most investors grumbled, though some were happy especially since the stock surged.