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.
Facebook has grappled with months of scrutiny over Russian misuse of the platform in the 2016 USA presidential campaign and the harvesting of its users' data through the political consulting firm Cambridge Analytica.
Facebook's tally of second-quarter global daily active users, a key metric for tracking user engagement, rose 11% to 1.47 billion.
Wehner attributed the poor revenue expectations to several factors: currency headwinds, the R&D cost of building and promoting new experiences, and impacts on Facebook's business from new privacy regimens in Europe due to GDPR.
The revenue shortfall reflects warnings from Chief Executive Mark Zuckerberg that Facebook's bottom line could be affected by costly measures it is taking to improve safety and security for its 2.23 billion monthly active users.
According to marketing agency Merkle, ad impressions on Instagram have also grown significantly, more than tripling year-over-year during the second quarter, with ad spending up 177% year-over-year among the agency's North America clients. However, ad sales seemed unaffected by the scandal, rising 42% in the quarter to $13bn (£9.9bn).
Facebook's 2.23 billion monthly active users and 1.47 billion daily active users were both up 11 percent from a year ago but also just shy of analyst predictions of 2.25 billion and 1.48 billion, respectively. Many Facebook analysts hope that Instagram's growth will help offset any stagnation at the older social network.
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The second-quarter results were the first sign that a new European privacy law and a succession of privacy scandals involving Cambridge Analytica and other app developers have bit into Facebook's business.
Facebook's daily active users for the quarter were 1.47 billion, shy of the 1.49 billion forecast by StreetAccount and FactSet.
Facebook's shares recovered somewhat after the call. It recorded sales of $13.23 billion for the three months ended in June, short of the $13.3 billion Wall Street anticipated. So far, WhatsApp doesn't show ads, and its founders Jan Koum and Brian Acton left Facebook amid disagreements over advertising and other issues.
As Wall Street analysts were expecting stronger user growth to 1.49 billion and revenues of $13.3bn, the company's stock price fell precipitously during an earnings call.
The problem: weaker-than-expected revenue growth, Facebook's first such miss since 2015.
In late May, the General Data Protection Regulation came into effect in Europe, restricting the way advertising companies collect data from individuals.
The threat of additional regulatory setbacks remains a concern, according to analysts.
Almost all social media services have received greater scrutiny since United States intelligence agencies in January 2017 revealed that organisations tied to the Russian government had seeded content on the platform to shake up the 2016 U.S. presidential election.