Factors contributing to Facebook's slowing sales growth include currency-exchange rates, compared with "the tailwinds we have experienced over the last several quarters", Wehner said.
Mr. Zuckerberg will be able to take comfort from the fact that Apple is now valued at more than three times the amount it was after its $60bn blip while Exxon is trading around 30 per cent up on its price in October 2008. But the results followed a period in which data-privacy issues came under harsh scrutiny, with Zuckerberg testifying before Congress for hours on the company's missteps. And the increased spending aims, among other things, to prevent a replay of the fake news and propaganda that Russian agents unleashed on an unguarded Facebook in an attempt to sway the 2016 presidential election.
Mark Zuckerberg's loss of $15.8bn is equal to the total wealth of the world's 81st-richest person.
Poletti said the steep decline was "stunning not only because of the scale, but because Facebook had managed to avoid this type of punishment through a multitude of sins too numerous to fully list here".
As to why the stock dropped so sharply, analyst Gene Munster said the stock drop "makes sense given what they revealed [Wednesday]".
The social media platform's second quarter earnings report showed it had lost approximately a million monthly users and 3 million daily users in Europe, with no user growth in the US. Leading up to the new stricter privacy rules, Facebook notified users across Europe of pending changes to its privacy policies.
US and Canadian Facebook visitors provided an average revenue per user (ARPU) in the latest quarter of $25.91, the vast majority from advertising, while the ARPU of Europeans was $8.76, according to figures provided by Facebook.
Of 47 analysts covering Facebook, 43 still rate the stock as "buy", two rate it "hold" and only two rate it "sell".
One bright spot for Facebook has been Instagram, the photo-sharing app it bought for $1 billion in 2012.
Until now, he said, there was a sense that the vast majority of users didn't fully understand Facebook's business.
It's the second time this year that Facebook's stock has taken a dramatic plunge. Facebook warned that its operating margins would fall from its current 44 percent to the "mid-30s".
"Unlike Netflix, whose quarterly shortfall we saw as temporary, here we see an evolution of the story, albeit a portion of which we expected", said Daniel Salmon, an analyst at BMO Capital Markets.
"The company has a track record of resetting revenue growth and expense expectations only to turn around and exceed those expectations the following quarter", Munster wrote.
Facebook shares fell more than 23 percent in after-hours trading on the poor earnings report, slumping at one point to $166 a share.