Netflix Inc. stunned Wall Street by attracting fewer subscribers than expected last quarter, renewing concerns that the video-streaming service has become an investment bubble. The shares plunged as much as 15 percent after Netflix said it added 5.2 million users in the period, about a million fewer than it predicted. The stock pared losses before the U.S. market open on Tuesday, trading down 12 percent as of 6:18 a.m. in New York. Netflix\u2019s outlook for the current quarter also reflected a deceleration: The world\u2019s largest paid online TV network expects to add 5 million customers, a slower pace than a year earlier. Shareholders and analysts now have the job of weighing whether the slowdown is a blip or a longer-term problem. Netflix\u2019s stock had more than doubled this year, with investors betting that the company will add tens of millions of customers around the world for years to come. Along the way, Wall Street may have focused more on the allure of the Netflix story, rather than the company\u2019s fundamentals, said Rob Arnott, head of fund advisory firm Research Affiliates. \u201cThey qualify as a bubble,\u201d he said on Bloomberg Television. The stock\u2019s slide to $353.70 erased about $21 billion in market value. Netflix executives expressed little concern on a call with analysts and investors, insisting their growth over the past 12 months has still exceeded expectations. Netflix fell short of its forecasts a couple years ago, a shortfall the company blamed at the time on the transition to chip-based credit cards. \u201cWe never did find an explanation to that other than lumpiness in the business,\u201d Chief Executive Officer Reed Hastings said. One reason for this shortfall may be a lack of content. Netflix released a thin slate of shows in the quarter, relative to its typical output. It didn\u2019t add additional seasons of its biggest hits, such as \u201cStranger Things,\u201d nor did a new show become a phenomenon. Ever since Netflix released \u201cHouse of Cards\u201d in 2013, the company has credited new seasons of original series with luring customers. Netfllix did release a new season of \u201c13 Reasons Why\u201d and the Marvel series \u201cLuke Cage,\u201d as well as a breakout standup comedy special in Hannah Gadsby\u2019s \u201cNanette.\u201d World Cup Woes? Potential new customers may have also been distracted by the World Cup, a quadrennial soccer tournament that is among the most-watched TV events in the world. Investors value Netflix at a far higher level than other media companies of similar size because of that potential for future growth. Its market valuation surpassed that of Walt Disney Co. this year, despite reporting less than a quarter of the revenue. Netflix\u2019s second-quarter revenue also came in short of projections. It posted $3.91 billion, compared with an average estimate of $3.94 billion. But the Los Gatos, California-based company hit a milestone: International customers accounted for a bigger piece of sales than domestic users. Once primarily a service for English speakers, Netflix has ramped up its investment in shows filmed in other languages. The company debuted its first Danish and first Indian dramas in the quarter, and it plans to release a new foreign-language program at least once a week next year. Profit was a bright spot in the latest quarter, but not by enough to reassure investors. Earnings amounted to 85 cents a share, topping the 79-cent estimate of analysts. \u201cWe had a strong but not stellar quarter,\u2019\u2019 the company wrote in a letter to shareholders. Spending Spree Producing and promoting a library of shows for a global audience has come at a high cost. Netflix has borrowed money repeatedly to pay for its programming, and expects to spend between $3 billion and $4 billion more in cash than it will generate in 2018. Marketing expenses surpassed $500 million in the quarter, nearly double the amount spent a year ago. Netflix\u2019s rise has pushed other technology and entertainment companies to invest more in online video services. Disney is selling an internet version of its sports network ESPN and plans to introduce a general entertainment video service next year. Apple Inc., meanwhile, is spending more than $1 billion on original programming. Netflix said Monday that it expects more competition, but dismissed any potential negative impact on its business. \u201cOur strategy is to simply keep improving, as we\u2019ve been doing every year,\u2019\u2019 the company said. Research Affiliates\u2019 Arnott expects the Netflix rout to have a broad impact on indexes. It had been the second-biggest gainer on the S&P 500 so far this year.\u201cThere\u2019s a distinct risk of a ripple effect,\u201d Arnott said.