By Sruthi Ramakrishnan and Subrat Patnaik
(Reuters) - Shopping through mobile devices soared on Cyber Monday, accounting for more than a quarter of the $3 billion in sales, but many online retailers struggled to get visitors using smartphones and tablets to shop as much as those using desktops.
Bigger-than-expected discounts and strong demand for electronics and toys made Cyber Monday the biggest ever day for online sales in the United States, according to the Adobe Digital Index report.
The report was based on data from 200 million visits to 4,500 retail websites on what is traditionally the busiest day of the year for Internet shopping.
Shopping using mobile devices accounted for 26 percent of total online sales, up from 19 percent a year earlier, Adobe's report said.
IBM's Watson Trend report said mobile traffic accounted for 47.9 percent of all online traffic, compared with 41.2 percent last year.
Top-selling items included Lego's Star Wars collection, the Barbie Dream House, Samsung (KS:005930) 4K television sets and Apple Inc's (O:AAPL) iPad Mini.
Star Wars toys were among the items most frequently out of stock, Adobe said.
The average order value from online shoppers was $123.43, down 0.6 percent from last year, IBM (N:IBM) said, mainly due to a rise in mobile shoppers.
On an average, smartphone shoppers spent $102.02 per order, well below the $128 spent by desktop users.
Even though there was a big increase in usage and purchases, conversion rates were "depressing," said Scot Wingo, executive chairman of ChannelAdvisor.
Only 2.19 percent of visits to websites resulted in a sale, compared with 2.18 percent last year.
"The fact that we haven't improved the mobile conversion rate is a little depressing to be honest," Wingo said.
Cyber Monday refers to the Monday after Thanksgiving during which shoppers return to work, where many have high-speed connections.
As the tradition of Cyber Monday has grown over the last decade, online retailers have offered bigger promotions. The average discount on items sold during Cyber Monday was 21.5 percent, Adobe (O:ADBE) said in its report.
Among many strong performers, Amazon.com Inc (O:AMZN) stood out, clocking a 21.1 percent rise in Cyber Monday sales, according to e-commerce software provider ChannelAdvisor.
Discounts on Amazon.com averaged 40 percent, e-commerce analytics company Clavis Insight said.
Online sales were in stark contrast to the performance of brick-and-mortar retailers over the four-day weekend. Sales fell 10.4 percent from a year earlier to $20.43 billion, according to research firm ShopperTrak.