It is among the biggest single-day falls in United States stock market history.
That "bombshell", as one analyst termed it, played into concerns on Wall Street that Facebook's model could be under threat after a year that has been dominated by efforts to head off concerns over privacy and fake news.
The number of daily active users in Europe fell by 3 million people from the first quarter, dropping to 279 million.
Media analyst Michael Nathanson raised the possibility of Facebook engaging in "a very public act of self-immolation" to stave off regulation, both in the US and in Europe, which just imposed tougher new privacy rules.
The Facbook CEO also allowed the Russian meddling scandal to develop in 2016 in which Russian operatives bought Facebook ads to influence USA voters.
That could be down to the drop being a one-off, potentially following the Cambridge Analytica data sharing scandal, or even the sharing of private posts belonging to some 14 million Facebook users, rather than a downward spiral for Facebook. But the results followed a period in which data-privacy issues came under harsh scrutiny, with Chief Executive Officer Mark Zuckerberg testifying before U.S. Congress for hours on the company's missteps.
"We continue to focus our product impact on putting privacy first, and that's going to have some impact on revenue growth", said Facebook chief financial officer David Wehner on the earnings call.
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Slowing revenue growth initially pulled the stock down almost nine in after-hours trading on Wednesday before losses picked up on the margin outlook. The social network missed analysts' expectations of $13.36 billion in revenue, while Q2 2018 marked its slowest ever user growth rate.
"Nobody knows where the top is, where that growth slows down", said Phil Bak, CEO of Exponential ETFs and a former managing director of the New York Stock Exchange who said he's been warning investors of a potential sell-off in large tech stocks.
Media push Facebook to censor Fox News! They believe users have woken up to Facebook's growing list of controversies and chose to vote with their "Log Out" buttons.
Of 47 analysts covering Facebook, 43 still rate the stock as "buy", two rate it "hold" and only two rate it "sell".
The site also brought in 1.47 billion global daily active users (DAUs) during its second quarter, below the 1.49 billion that analysts were projecting, according to a StreetAccount and FactSet outlook.
All of this was enough to spook investors in the shares on the Nasdaq when the market opened on Thursday morning.