HONG KONG — Singles Day in China has become the world’s biggest shopping day. Yet its most impressive aspect is not the millions of packages shipped or the billions of dollars spent, but the fact that it exists at all.

With clever marketing and help from an online spending boom, the Alibaba Group, in less than a decade, turned Nov. 11 into a symbol of the power of the Chinese consumer. By the end of Friday, by Alibaba’s count, more than $17.7 billion had changed hands on its platforms — exceeding sales from last year’s event, but a marked slowdown in growth from the previous year.

Yet the very dynamics that helped Alibaba engineer the Hallmark-like holiday out of thin air have turned against it.

Growth in new internet users is slowing in China, as is growth in the number of Chinese people and companies opening stores on Alibaba’s sales platforms. More important, growth in the value of the goods sold on the company’s platforms is decelerating.