NEW YORK — Cyber Monday started as a gimmick to get people to shop at their desks on the first workday after Thanksgiving. But if you promote something enough, it can take on a life of its own. This year, stores swamped customers with online ads and email deals, and sales could top $1 billion, making it bigger than any single shopping day last year. Online sales were already running 15 per cent ahead of last year’s by 3 p.m. Monday, with the biggest shopping hours of the day still to come, according to IBM’s Coremetrics tracking service. “The numbers are really strong,” said the service’s chief strategy officer, John Squire, who added that he expects Cyber Monday to be the biggest online shopping day of the season.