ComScore (, a company that “measures the digital world,” has reported holiday season U.S. retail e-commerce spending for the first 28 days of the November- December 2011 holiday season. For the holiday season-to-date, US$15 billion was spent online, marking a 15% increase versus the corresponding days last year.

Cyber Monday reached $1.25 billion in online spending. That’s up 22% compared to last year, representing the heaviest online spending day in history and the second day on record to surpass the billion-dollar threshold.

“Cyber Monday was yet another historic day for e-commerce, with online spending reaching a record $1.25 billion,” says comScore chairman Gian Fulgoni. “It was just the second billion dollar spending day on record, following on the heels of Cyber Monday 2010. While last year saw Cyber Monday rank as the heaviest online spending day of the year for the first time ever, it will be interesting to watch the next couple of weeks to see if any future individual days in 2011 manage to leapfrog this year’s highest day-to-date.”

Cyber Monday’s 22% growth in sales versus year ago was driven by an increase in both the number of buyers (up 11%) and the average spending per buyer (up 9%). Overall, 10 million people bought online on Cyber Monday, representing the first time on record that threshold has been reached in a single day. The average online buyer conducted 1.9 online transactions on Cyber Monday for a total of nearly $125 in spending.