James Martin/CNET

Investors could finally be losing their patience with Amazon.

Amazon's stock tumbled 11 percent in early trading Friday to about $318 a share, after the company reported a wider-than-expected second-quarter loss. As of Thursday's close, the stock was already down 10 percent so far this year.

The online retailer has enjoyed an incredible run-up in its stock for years, even as it has spent heavily to fuel rapid growth, eschewing profits along the way. In past quarters, investors cheered strong revenue growth despite tiny profits or even losses.

Amazon's stock now is experiencing a sell-off after the company came out with a second-quarter loss of $126 million on Thursday. An even bigger operating loss, as much as $810 million, is predicted for the current quarter.

To quickly grow, Amazon has taken the money it may have booked as profits and instead spent it on more warehouses, employees, and technology, while also undercutting prices of some competitors. Common investor wisdom has stated for years that Amazon's high-spending, low-pricing model would force competitors out of the market, allowing the online retailer to eventually raise prices, cut back on spending, and start posting strong profits. That scenario still hasn't played out.

Instead, Amazon is still driving up its spending, adding to its hardware line with the new Fire Phone and investing in original content for its streaming-video service in an effort to rival Netflix.

Amazon could also end up facing increased competition from Chinese e-commerce juggernaut Alibaba, which is planning one of the biggest initial public offerings in US history this year and is expected to push into the US market.

Overall, Amazon on Thursday posted a quarterly loss of $126 million, or 27 cents a share, compared with a year-earlier loss of $7 million, while revenue jumped 23 percent to $19.34 billion. Analysts on average expected the Seattle-based e-commerce giant to report a loss of 15 cents a share with $19.34 billion in revenue. Total operating expenses rose 24 percent to $19.35 billion, just above total revenue.

RBC Capital Markets analyst Mark Mahaney said in a note Friday that Amazon wins the award for "most aggressive investor in the Internet sector," with its expansion in China, groceries, faster delivery, video content, and consumer staples.

Although RBC lowered its price target for the company, it reiterated its outperform rating. Mahaney explained that Amazon has proven before that it can be profitable, and its higher expenses are part of the company's offensive posture, rather than a defensive one. Ultimately, he said it's likely at least some of this super-aggressive investing will pay off in the form of higher revenue and better margins.

The company is certainly doing a lot of things as it tries to gain an edge on other online retailers such as eBay and brick-and-mortar players like Wal-Mart and Target. It seems the one thing it may not be doing is pleasing investors.

Update, 8 a.m. PT: Adds analyst's comments and updates stock price.