By midday, total e-commerce sales are up 13.3 percent year-over-year, which is slightly below the 15 percent growth rate expected by ChannelAdvisor.
The e-commerce sales tracker says the total is being pulled down the most by digital marketing channels Google Shopping, CSE and search.
But Cyber Monday sales tend to ramp up through the day, so ChannelAdvisor posits that at its current pace, this year's event is still on track to break records and become the largest online sales day of 2015.
As for how the sales breakdown across key marketplaces, there's one very clear leader of the pack: Amazon.
The e-commerce giant is cleaning house, with sales up 23 percent year-over-year.
By contrast, eBay is up just 1.9 percent year-over-year and Google Shopping is up 14.3 percent. All other third-party marketplaces have combined growth of 46 percent year-over-year.
Additional data from IBM puts the Cyber Monday online sales forecast at more than 18 percent after jumping 26 percent on Saturday and Sunday. Using data from the IBM Watson Trend App, IBM said the increase is being driven by a surge in mobile shopping as well as demand for select products.
The average online order so far on Monday was $127.56, with 30 percent of all sales coming from smartphones and tablets, IBM said.
Eventually Black Friday may overtake Cyber Monday as the top performing online sales day, but Cyber Monday still commands a lead over Black Friday and Thanksgiving. Mostly it boils down to convenience and whether retailers will continue to spread online sales deals throughout the Thanksgiving holiday week.