Facebook's owner Meta Platforms saw its stock market value slump by more than $230bn (£169bn) on Thursday, in a record daily loss for a US firm.
Its shares fell 26.4% after quarterly figures disappointed investors.
Meta also said that Facebook's daily active users (DAUs) had dropped for the first time in its 18-year history.
The company's share price slide saw chief executive Mark Zuckerberg's net worth fall by $31bn, according to the Bloomberg Billionaires Index.
The drop in Mr Zuckerberg's personal fortune was equivalent to the annual gross domestic product of Estonia.
Even after that drop, Mr Zuckerberg has an estimated net worth of almost $90bn, which means he is still one of the richest people in the world.
That came after Meta revealed that Facebook's DAUs fell to 1.929bn in the three months to the end of December, compared to 1.930bn in the previous quarter.
It was the first time ever that this measure of activity on the world's biggest social network had gone into reverse.
Meta's stock market slump came on the eve of the 18th anniversaryMeta also warned of slowing revenue growth in the face of competition from rival platforms including TikTok and YouTube, while advertisers were also cutting spending.
Mr Zuckerberg said the firm's sales growth had been hurt as audiences, especially younger users, had left for rivals.
The firm forecast revenues of between $27bn and $29bn for the first quarter of this year, which was lower than analysts had expected.
Although the company has been making investments in video services to compete with TikTok, owned by Chinese technology giant ByteDance, it makes less money from those offerings than its traditional Facebook and Instagram feeds.
nding of Facebook