Retailers, take heed. The numbers are in for Cyber Monday, and one thing came out loud and clear: it is all about smartphone sales.

As of 4:30 p.m. ET, Monday, Nov. 27, Cyber Monday sales reached $3.38 billion, reflecting a 16.8% growth from last year and marking the biggest day in U.S. e-commerce history. The kicker is that smartphone visits make up more than half the overall traffic Monday, according to data from 10 a.m. 

Mobile represented 53.3% of digital sales as of 10 a.m. Cyber Monday, according to Adobe Analytics data. That's 44.6% from smartphones and 8.7% from tablets. Mobile sales account of nearly 40% of total revenue, and mobile revenue grew 41% year over year. 

By far, the most surprising metric in this year's data is mobile sales, according to Tamara Gaffney, senior director of Adobe Digital Insights.

"Today in theory should not be a big smartphone day because people are back at their desktops," Gaffney told TheStreet Monday. "The most interesting thing I see in this data is that this isn't the case."

The former inconvenience of online shopping boiled down to what she calls "fat finger problems." Thanks to new technologies like autofill, buying mobile is as easy as, literally, just a few clicks.

Credit card autofilling on mobile, it turns out, is a new development in browser technology. It allows browsers such as Google Chrome to store payment details the way they do with passwords and apply them to each online purchase. Called Payment Request API, this capability replicates the technology of payment providers like PayPal Holdings Inc.

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and Inc.

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, according to tech support website Bleeping Computer.

Social media functions may have also contributed to the trend. Instagram's click-to-buy function, for instance, allows users to shop directly within the app. Owned by Facebook, Inc. (FB) - Get Report , the Instagram technology was rolled out to clothing, jewelry and beauty brands in March.

For retailers, the numbers indicate that having the best mobile experience is now more important than ever, Gaffney said. Those numbers also point to the progress that they've made, according to John Roswech, executive vice president of brand solutions at Criteo SA (CRTO) - Get Report , a commerce marketing platform that tracks digital sales. 

"From the numbers, we see that basically more sites have done a fabulous job of optimizing for mobile and making their sites responsive," he told TheStreet. "The [autofilling] probably helped and ... people are also doing more research before getting online, which may have increased the conversion rates." 

While it may be surprising that so many people are opting to shop on their phones rather than computers, it's no news that Americans are spending more and more time on their phones. A March report from web analytics firm Flurry found that consumers are spending up to five hours a day on mobile devices — up 69% since last year. More than half of that time is spent on social media.

Cyber Monday followed Thanksgiving and Black Friday trends, in which mobile also made up a large fraction of sales. Mobile made up 40% of transactions on Black Friday, according to Criteo's data — that's up from 29% last year. Meanwhile, preliminary numbers from ShopperTrak found that foot traffic at brick-and-mortar stores dipped 2% on Black Friday and Thanksgiving. 

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