Bezos noted the milestone in his annual shareholder letter, published Wednesday. The letter is the founder's opportunity to underline his long-term strategy for investors, seeking to bolster their confidence as he continues to plow Amazon's money into expanding internationally, building a brick-and-mortar presence and inventing products such as Echo speakers and the Alexa voice-activated digital assistant.

Bezos cited Amazon's continued high marks from independent surveys, including the American Customer Satisfaction Index, as evidence that the company continues to invent new ways to satisfy customers, giving them new things they didn't realize they wanted.

"You cannot rest on your laurels in this world," he wrote. "Customers won't have it."

The CEO also said that in 2017, Amazon shipped more than 5 billion items with Prime worldwide and more new members joined Prime than in any previous year. He added that 2017 was the best year for hardware sales and that the company will continue to invest aggressively to expand its customer base, brand and infrastructure.

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