(REUTERS) -- Amazon.com Inc., eBay Inc., and other online commerce companies had a strong Black Friday, but the industry's growth rate lagged last year's surge in online buying on the traditional first day of the holiday shopping season.

Online shopping jumped 24.3 percent on Black Friday, the day after Thanksgiving, compared with the same period a year earlier, IBM Benchmark, which tracks transactions on the Web sites of 500 retailers, said on Saturday.

IBM was expecting growth of about 15 percent. Higher-than-expected online buying was partly driven by increased use of mobile devices, it said.

Some of the best-performing retailers also did a good job informing consumers about online promotions ahead of the crucial post-Thanksgiving shopping day, said John Squire, chief strategy officer at IBM Smarter Commerce.

Mercent, a company founded in 2005 that helps retailers sell through online commerce channels including Amazon, eBay, and Google Inc.'s Web search engine, reported a 23 percent jump in Black Friday same-store sales by clients.

E-commerce continues to grow on an absolute basis and take additional market share from (traditional) bricks and mortar retailers, said Eric Best, CEO of Mercent and a former executive at Amazon.

Best expects a huge Cyber Monday -- falling this year on Nov. 28 - but he warned that this doesn't mean the whole holiday season will be strong.

Cyber Monday is the first work day after Thanksgiving, when workers often use office computers to shop online. Last year, more than $1 billion was spent on Cyber Monday, making it the heaviest online shopping day in history, according to ComScore.

Holiday shopping is becoming more front-loaded around Black Friday and Cyber Monday, Mercent's Best said. All of this is great news, but it's still early in this 45-day shopping season.

Weather Effect

Last year, Mercent clients saw same-store sales growth of 30 percent on Black Friday. Best said the slower growth this year may have been caused by better weather.

The weather was really good this year, which can have an effect on the amount of screen time, Best added. People were outside shopping offline or they weren't shopping at all.

Amazon saw some of the strongest growth again this year, according to ChannelAdvisor, which helps merchants sell on Web sites run by Amazon, eBay, and other online commerce companies.

ChannelAdvisor clients saw same-store sales via Amazon jump 49.8 percent on Black Friday, versus a year earlier.

Traditional retailers offered Black Friday deals earlier this year, but Amazon aggressively matched this strategy by running Black Friday promotions online starting on Nov. 21, according to Scot Wingo, CEO of ChannelAdvisor.

Still, on Black Friday in 2010, ChannelAdvisor clients saw same-store sales growth of about 80 percent via Amazon, Wingo noted.

Sellers on eBay generated same-store sales growth of 15.1 percent on Black Friday this year, down from 15.5 percent year-over-year growth in 2010, he added.

Mobile Shopping Surges

Mobile shopping surged on Black Friday, but remained a relatively small part of overall online commerce.

EBay said U.S. shoppers bought almost two-and-one-half times as many items via mobile devices this past Black Friday compared to 2010.

PayPal, eBay's online-payments business, saw a sixfold increase in global mobile-transaction volume on Black Friday, versus the same day last year.

More than 14 percent of Black Friday traffic on retail Web sites came from mobile devices, compared with 5.6 percent a year ago, according to IBM.

EBay said it offered a Microsoft XBox 360 250GB Kinect gaming system for $259.99 the evening before Black Friday that sold out in 20 minutes -- at a rate of 75 units per minute.

Best-sellers at Amazon on Black Friday included the company's Kindle Fire tablet computer, a Sony Cyber-shot DSC-TX10 Waterproof Digital Camera, and a Mr. Coffee Single Serve coffee machine powered by Keurig, according to an Amazon representative.

(Reporting by Alistair Barr; Editing by Paul Simao)