The Netflix logo on an iPhone in Philadelphia in 2017.
Netflix signed up 4.47 million subscribers internationally, while analysts were expecting 4.97 million.
The numbers released on Monday are a rare letdown for a company that has enthralled investors with its ability to consistently top expectations.
Netflix shares lost about $50 or 12 per cent in after hours trading. The shares had more than doubled before the sell-off.
The company's stock closed regular trading up $4.68 a share, or 1%, to $400.48 and was up almost 150% over the a year ago.
In a research note, GBH Insights analyst Daniel Ives called the second-quarter showing "a near-term gut punch" to Netflix. The streaming-video company reported Monday that it added 5.2 million new members between April and June, a million fewer customers than it had previously forecast.
USA arrests, accuses woman of acting as Russian agent
In truth, Helson wrote, "Butina continued to act as an agent on behalf of the Russian Federation" and an unnamed Russian official. In May 2016 , GOP operative Paul Erickson allegedly tried to get Butina and Torshin a meeting with then-candidate Donald Trump.
Netflix added 670,000 subscribers in the United States, well below analysts' estimates of 1.19 million, according to Thomson Reuters I/B/E/S.
The spring and summer months traditionally mark Netflix's most sluggish period as more people go on holiday and spend time outside instead of watching video. Netflix, based in Los Gatos, California, didn't specify why it fell short last quarter, beyond citing the difficulty of forecasting growth in 190 countries around the world. Earnings grew 32 percent from previous year to $384 million, or 85 cents per share. For the first time, Netflix generated more revenue outside the U.S.: global revenue totaled $1.92 billion and U.S. revenue was $1.89 billion for Q2.
Wall Street had been betting that Netflix would deliver outsized growth as demand for online entertainment increases around the globe.
Netflix is still expecting a big second half of 2018 with many new hit shows still to come and several big deals.
But it also faces growing competition. And Walt Disney is hoping to close on a $71 billion deal to buy prized entertainment franchises from 21st Century Fox to feed into a video streaming service Disney will debut next year. Netflix has a strong slate of original content that should keep it in the forefront among streaming services, and it plans to continue outspending the competition to develop TV programming and feature films.