IMAGE DISTRIBUTED FOR EBAY - The busiest mobile shopping day on eBay in the U.S. was Sunday, December 15. Sunday’s have been the most popular day for iOS users to shop on eBay with the most popular categories being Fashion, Collectibles, and Electronics. (Photo by Paul Sakuma Photography/eBay via AP Images)
There's a mobile app for eBay's smartphone consumers. (Photo by Paul Sakuma Photography/eBay via AP Images)

For years now, we've heard a steady drumbeat of forecasts suggesting that mobile devices are going to play an increasingly crucial role in every step of a shopping expedition, from browsing and couponing to purchasing.

And in the five-day shopping bonanza that stretched from Thanksgiving to Cyber Monday, there was ample evidence that this long-anticipated tipping point has arrived.  For the first time ever, smartphones and tablets accounted for the majority of all online traffic on Thanksgiving Day, according to data analysis by IBM. On Black Friday, one in four online purchases were made on a mobile device.

"This year was probably the first holiday season that we really saw mobile really take off,” said Brian Yarbrough, senior retail analyst at financial services firm Edward Jones.

Wal-Mart says that about 70 percent of its Web traffic between Thanksgiving and Cyber Monday came from mobile devices, while JCPenney said more than half of its traffic came from smartphones and tablets. Best Buy's Web site was briefly taken down Friday morning while the company struggled to deal with an unexpected surge in mobile Web traffic.

As users take to mobile platforms in greater numbers, retailers are learning that small-screen shoppers are different: They buy fewer things and have less patience for a checkout process that is anything less than incredibly efficient.

Also, while shoppers did plenty of browsing this holiday season on their cellphones, tablets and PCs still reigned for actually making purchases. Smartphones accounted for 34.7 percent of traffic on Black Friday, but just 11.8 percent of sales.  Tablets accounted for 14.6 percent of traffic and 16 percent of sales.

"There's some kind of a disparity between traffic and conversion on smartphones," said Calvin Silva, senior retail industry analyst for Nasdaq Advisory Services.

BaubleBar discovered that how mobile customers reach the e-commerce jewelry company's site can make a difference. Tablet shoppers were more likely to reach the company's site through a search engine this holiday season, while smartphone users were more likely to come from Facebook or other social media.

The company had to adjust, said Daniella Yacobovsky, a BaubleBar co-founder. For tablets and laptops, which have larger screens, Yacobovsky said, "we feel comfortable sending them to pages that have a little bit more of a design element to them."

Elissa Margolis, senior vice president of Disney Store North America, said that a large share of Disney Store's mobile shoppers were simply browsing. "Our merchandise is exclusive, so they’re not using it [for] comparisons across other retailers," Margolis said. "But they may be using it to see if an item is available, read reviews."

Margolis said that about 90 percent of Disney Store's mobile traffic has typically come from Web browsers, not the store's app. But Margolis said the company is now doubling down on trying to draw more users to its app, which she acknowledges will be challenging.

"There needs to be a reason why you would put an app on your phone versus just using your browser," Margolis said.

JCPenney revamped its iPhone app and mobile Web site in time for the holidays.  The company said it believes the more seamless experience helped it turn more mobile browsers into buyers, reaching a conversion rate "in the 30 percent range" during parts of the holiday weekend.

Retailers and analysts say they expect that the growth of mobile sales and traffic to be a bright spot throughout the holiday season. In particular, they predict these gadgets will be crucial weapons for the omnichannel shopper -- that is, the one who bounces around between in-store, desktop and mobile shopping.

"The consumer expects to be able to go in and out of a physical and digital world in a completely seamless way," said Doug Hope, a retail marketing expert with GlobalShop. "This is the very beginning of a very significant shift in focus."