US consumers spent $30.9 billion toward online purchases during the current holiday season, a 15 per cent increase from the year-ago period, as shoppers took advantage of free shipping and other online deals, according to Web-tracking firm Comscore.

Comscore also said spending has reached a crescendo for this season and will begin to slow as Christmas nears.

Sales surpassed $1 billion on four days during the work week of December 12th to December 16th, said comScore, a closely watched Web-tracking firm.

"Free shipping is undoubtedly one of the most important incentives for consumers and has become a key driver of online buying activity over the past few years," Comscore chairman Gian Fulgoni said in a statement.

Comscore said "Cyber Monday" has become the top-ranked shopping day for the second year in a row. Cyber Monday is traditionally the first Monday after Thanksgiving when employees return to offices and purchase items with their work computers.

JC Penney and other department stores, including Kohl's and Macy's, used Facebook a lot more this year to attract online shoppers.

JC Penney released its Black Friday, the day after Thanksgiving, deals on Facebook, allowing customers to browse the promotions, create shareable wish lists and send tips to friends.

Comscore said it measured the data for the first 46 days of the November-December 2011 holiday season.

Reuters