• by Laurie Sullivan  @lauriesullivan, November 8, 2013

An unofficial assessment for Q4 2013 mobile commerce could see the total climb to $9 billion in the United States -- up from comScore's $5.8 billion estimate for sales from tablets and smartphones in the prior quarter.

No official forecast yet on mcommerce, but Andrew Lipsman, VP of marketing and insights at comScore, said he expects it to come close to "$9 billion, with an outside shot of hitting $10 billion."

comScore Friday said Q3 2013 U.S. desktop-based retail e-commerce sales grew 13% year-over-year to $47.5 billion, marking the sixteenth consecutive quarter of positive year-over-year growth and the twelfth consecutive quarter of double-digit growth.

Mobile commerce spending on smartphones and tablets grew 26% compared with the year-ago quarter, for a total of $53.2 billion in Q3 for digital commerce spending. Of the additional $5.8 billion in mcommerce, purchases from smartphones accounted for 62% versus 38% from tablets.

E-commerce accounted for 9.4 percent of consumers' discretionary spending, the highest third-quarter share on record. comScore plans to preview insight into Q4 2013 desktop and mobile commerce, along with holiday trends, during the State of the U.S. Online Retail Economy through Q3 2013.

In Q3 2013, top-performing online product categories were Digital Content & Subscriptions, Apparel & Accessories, Consumer Packaged Goods, Consumer Electronics and Jewelry & Watches. Each category grew at least 14 percent versus a year ago.

A PM Digital report cites ecommerce as a driving factor for holiday sales, although the amount of organic traffic leading to conversions on brand Web sites continues to slip. Organic traffic share fell to 22.20% of the total, while organic search revenue slipped from 21.32% to 18.58% in Q3 2013.

Twitter's scheduled and organic tweets, and Instagram paid posts, however, are tools that will help search marketers drive awareness up the funnel and increase purchase conversion rates through the final quarter of 2013.

Happy women shopping photo from Shutterstock