In a bright spot for China's cooling economy, online shoppers spent billions of dollars Friday on "Singles Day," a quirky holiday that has grown into the world's busiest day for e-commerce.

The country's biggest e-commerce brand, Alibaba Group, said sales by the thousands of retailers on its platforms passed 91.2 billion yuan ($13.4 billion) in the first 15 hours of the event. That is four times the $3 billion research firm comScore says Americans spent in total last year on Cyber Monday, the country's biggest online shopping day.

Rivals including JD.com, VIP.com and Suning offered deep discounts on clothing, smartphones, travel packages and other goods to attract shoppers.

JD.com, the country's biggest online direct retailer and Alibaba's top rival, said it tested delivery by drone to customers in four rural areas in what the company believed to be the first commercial use of such service. The company said its sales passed last year's Singles Day total at 1:33 p.m. but gave no financial amount.

Singles Day was begun by Chinese college students in the 1990s as a version of Valentine's Day for people without romantic partners.

The Nov. 11 date was picked to be "11.11"--four singles. Young people would treat each other to dinner or give gifts to woo that special someone and end their single status.

The spending gives a boost to the ruling Communist Party's efforts to nurture consumer-based economic growth and reduce reliance on trade and investment.

E-commerce sales in China rose by 26.1 percent in the first nine months of the year. Economic growth for that period held steady at 6.7 percent, but that was its lowest level since the 2008 global crisis.

Forecasters expect the economy to cool further next year as regulators try to rein in a boom in bank lending and real estate sales that is pushing up debt levels and housing costs.

China has the biggest population of Internet users at 710 million, according to government data. Some 410 million people shop online for goods ranging from clothing and groceries to manicures and plane tickets.

"Online shopping is getting more and more common," said He Mei, an employee of a health products company in her 30s who had waited for Friday to buy an indoor air filtering machine at a discount.

"Young guys, especially those in their 20s, don't really go out to buy things, and they buy pretty much everything online," she said. "It's so easy and it saves time and money."

The migration of Chinese consumers to online commerce and entertainment is squeezing traditional retailers, cinemas and other businesses, forcing them to improve service and add offerings.

E-commerce has risen from 3 percent of Chinese consumer spending in 2010 to 15 percent last year, according to Boston Consulting Group. It forecasts online spending will rise by 20 percent a year, hitting $1.6 trillion by 2020, compared with 6 percent growth for off-line retail.

Researchers attribute the rapid rise of Singles Day to demographics and timing.

University graduates who adopted the holiday earn more and shop online. Also, Singles Day comes as people receive monthly paychecks and need to buy winter clothes.

Unlike other events such as the Lunar New Year, China's biggest family holiday, it involves few other expenses such as travel or banquets, leaving more money for gifts.

This year, Alibaba hired actress Scarlett Johansson, football star David Beckham, basketball legend Kobe Bryant and pop-rock band One Republic to for a pre-sale gala that was broadcast online to drum up attention.