In midday trading Thursday, the company's market value (the number of outstanding shares multiplied by the value of a single stock), fell by more than $122 billion.
In addition, the company expects to invest in products like Stories "that now have lower levels of monetization", while the introduction of new controls for users to limit their data-sharing with Facebook "may have an impact on our revenue growth", the CFO said.
Sales in the second quarter grew 42 per cent, its slowest pace in almost three years, to US$13.2 billion compared with US$9.3 billion a year ago.
Trillium's proposal, if greenlit by investors including Facebook's own management, would require the company to appoint an independent chairman, breaking up Zuckerberg's dual role as CEO and chairman.More news: Celebrities dazzle at Akash-Shloka's engagement party
Amazon's shares are worth $2,200, according to JPMorgan Chase, with analysts boosting their 12-month price target on the company to $2,200 per share from $1,900. Shares, which had already declined 7 percent after hours, then fell as much as 24 percent after the comments on a conference call with analysts.
The value was down by even more overnight after the company posted quarterly results late Wednesday showing its growth was grinding to a halt in Europe and North America, at a time when increased privacy safeguards would raise its cost of business.
The complaint filed by shareholder James Kacouris in Manhattan federal court accused Facebook, Mr Zuckerberg and Chief Financial Officer David Wehner of making misleading statements about or failing to disclose slowing revenue growth, falling operating margins, and declines in active users. The stock was up more than 23 per cent for the year, before it reported earnings after Wednesday's close.
The company's second-quarter revenue - $13.23 billion - missed expectations by about $70 million.More news: Matteo Darmian wants to leave amid Juventus & Inter links, admits Jose Mourinho
"Our community and business continue to grow quickly", said Zuckerberg.
Facebook said it had 1.47 billion daily active users in June, compared with the 1.48 billion average of analysts' estimates compiled by Bloomberg.
"Looking beyond 2018, we anticipate that total expense growth will exceed revenue growth in 2019", he said.
It's a staggering drop-off for Facebook and flies in the face of Wall Street's expectations. European DAUs were down from 282 million last quarter, potentially related to the effect of the enactment of the General Data Protection Regulation (GDPR) in the European Union.More news: Pep Guardiola: 'Almost impossible' for Manchester City to break own records
Facebook used to be made out of corporate Teflon. All of this was enough to spook investors in the shares on the Nasdaq when the market opened on Thursday morning. "We've seen a greater-than-expected efficiency in a lot of our spend in things like warehouses, data centres, marketing". But perhaps most damaging of all, the company warned of a growth slowdown.