UPS did not immediately return a request for comment Thursday morning, as complaints via social media continued to pile up.
"I will never use UPS again... never!" Samuel Barnett wrote on the company's Facebook page after his product from Appledidn't arrive by Christmas, despite the fact he ordered it second-day air more than a week earlier.
The reactions weren't all bad, though.
Heidi Grant, a North Carolina mother of three who didn't receive her gifts in time, told TODAY, "I guess the benefit is Christmas is going to last longer. [The kids are] going to have more gifts to open in a couple of days."
And on Twitter, others blamed procrastination of disappointed customers who had taken to using the hashtag "UPSFail" to complain about not getting their packages.
"People using #upsfail need to get a life," wrote @MrsKimSanders. "Order early next year!! Thank you UPS workers!!"
On Facebook, interspersed with the gripes at UPS were messages of gratitude for the workers.
"I am grateful for UPS everyday of the year and a HUGE UPS fan, but seeing MANY trucks out last night around 6 on Christmas Eve, I was most grateful for you guys and prayed you all got home safe & sound soon after I saw you," wrote Kelley Mansfield.
Added Hana Johnson, "We have the best UPS delivery man and I applaud all the hours of work they have put in. Merry Christmas and thanks."
The last time a significant number of UPS packages were late for Christmas was in 2004, when an ice storm crippled the UPS distribution center in Louisville, Ky. Employees manually loaded packages for days that year, surprising customers with Christmas Day deliveries.
—By Elizabeth Chuck, NBC News; Tony Dokoupil contributed to this report.