(Bloomberg) -- Amazon.com Inc. is back in fashion with investors.

Concern about the cost of Amazon’s one-day delivery pledge lingered after a busy holiday period, holding the stock back while shares of rivals including Microsoft Corp., Google and Apple Inc. all rallied. That abruptly changed Thursday when Amazon reported results that crushed Wall Street estimates.

Chief Executive Officer Jeff Bezos added to the enthusiasm by revealing the company has 150 million Prime subscribers who pay monthly or annual fees for shipping discounts and other perks, up from 100 million about two years ago. The stock surged 10% in extended trading.

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The largest U.S. e-commerce company has ramped up spending in several key parts of its business. In e-commerce, the company is rolling out one-day delivery to fight Walmart Inc. and other retailers. The Amazon Web Services cloud business is building new data centers and hiring engineers in response to steady gains by Microsoft Corp. and a renewed customer push from Alphabet Inc.’s Google. Meanwhile, Amazon continues to plow money into overseas markets such as India and Brazil.

The results showed that these investments are paying off. Fourth-quarter sales climbed 21% to $87.4 billion and, profit rose to $6.47 a share, the Seattle-based company said in a statement. Analysts, on average, projected sales of $86.2 billion and earnings of $4.11 a share, according to data compiled by Bloomberg. Amazon’s forecasts for the current quarter matched expectations.

“I’m shocked by the profits,” said Tom Forte, analyst at DA Davidson & Co. “I thought the story was going to be how investments in next-day shipping pushed profits down.”

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The big increase in Prime subscriptions suggests Amazon is luring overseas shoppers as the U.S. market saturates. Prime members spend about twice as much as other customers.

“This shows how big of a step one-day delivery was in terms of maintaining engagement on its marketplace,” said RJ Hottovy, an equity analyst at Morningstar Inc. “The Prime membership number shows they are seeing strong growth internationally.”

There was good news for investors in nearly every part of Amazon’s business. Cloud-computing and online sales beat analysts’ expectations. Shipping costs grew at a slower pace than in the third quarter, showing Amazon is making its delivery network more efficient. The only issue was Whole Foods groceries, with revenue from physical stores declining marginally from a year ago.