Netflix Inc.'s subscriber growth fell short of Wall Street expectations on Monday, sending shares of the normally high-flying stock down 14 percent on fears that the company's expansion is slowing.

The company added 5.2 million customers from April through June, 1 million fewer than forecasts from Thomson Reuters I/B/E/S, as it added new programming including "Lost in Space" and new episodes of Marvel's "Jessica Jones" and "13 Reasons Why."

"We had a strong but not stellar Q2," Netflix said in a quarterly letter to shareholders.