Two of the nation's biggest package shippers hit the road Thursday, scrambling to deliver boxes to angry customers who didn't receive the gifts they ordered in time for Christmas.
UPS did not make pickups or deliveries on Christmas Day, but brought in extra workers on Christmas night to the company's hub in Louisville, Ky., to sort packages for Thursday and Friday deliveries, a spokesperson for the company told The Associated Press. Some FedEx customers were able to pick up packages at local FedEx Express centers on Christmas Day.
(Read more: UPS, FedEx struggle with delayed Christmas gifts)
Both companies, pummeled on social media by negative comments from furious, gift-less customers, issued apologies.
"The volume of air packages in our system exceeded the capacity of our network. ... We apologize," UPS said in a statement. FedEx echoed that apology, calling the volume an "extraordinary event."
Besides poor weather in many parts of the country, the delays were blamed on systems overloaded by a shorter holiday shopping period this year due to Thanksgiving falling so late in November and more present-buying done online.
Online spending jumped 9 percent, to $37.8 billion, between Nov. 1 and Dec. 15, according to online research firm comScore, and retailers expect overall holiday sales to be up nearly 4 percent, exceeding $600 million.
Neither UPS nor FedEx said how many of its packages were delayed, but said it was a small percentage of the Christmas shipments.
(Read more: Worst holiday sales since 2008? Not so fast)
Scott Fielder, a spokesman for FedEx, said FedEx handled 275 million shipments between Thanksgiving and Christmas.
"Our 300,000 team members were focused on safely delivering our customers packages this holiday season and we are proud of their efforts," he said in a statement Thursday. "We operated with very high service levels — over 99 percent at FedEx Ground, for example — during our busiest time of the year."
More from NBC News:
'Mother nature has not been kind': Power misery to continue for tens of thousands
Chocolate, beef, more: 5 items that'll cost you more in 2014
Online gifting: Plenty of happy and (sometimes) unhappy returns
Jay Erikson, a customer who took to FedEx's Facebook page to express his frustration, was one of the unlucky ones who didn't get his parcels on time.
"If you can't keep a promise, then don't make it. Thanks for nothing, FedEx," he wrote.
Amazon.com, one of the nation's largest package shippers, cited UPS's "failure" in an email to customers on Christmas morning. The online retailer said it processed all orders on time and said it is compensating some customers with $20 gift cards and refunds on shipping charges.
(Read more: Amid holiday delivery woes, Amazon limits Prime)
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.