Struck by unexpected volume, package delivery companies failed to deliver on their promises this holiday season.

Both UPS and FedEx were unable to ensure all gifts arrived at their destinations by Dec. 25, despite pre-holiday shipping guarantees.

On Wednesday, UPS posted the following service advisory to its website:

"UPS understands the importance of your holiday shipments. UPS is experiencing heavy holiday volume and making every effort to get packages to their destination; however, the volume of air packages in our system exceeded the capacity of our network immediately preceding Christmas so some shipments were delayed. UPS is not making pickups or deliveries on Christmas Day and will resume normally scheduled service on December 26. For the most-up-to date information, click here to track your specific shipment's progress on UPS.com."

In preparation for the holiday season, UPS hired more than 55,000 seasonal employees.

"The entire team at UPS has planned all year to handle the annual increased shipping volume during the holidays," Alan Gershenhorn, chief sales, marketing and strategy officer, said Dec. 16 in a company statement. "As consumers change the way they shop for holiday gifts and shipping needs shift, customers rely even more on UPS for trusted and convenient solutions around the globe."

FedEx also didn't deliver packages on Christmas, but did allow some customers to pick up orders at local FedEx Express centers.

"We're sorry that there could be delays and we're contacting affected customers who have shipments available for pickup," said Scott Fiedler, a spokesman for FedEx Corp., told AP.

The delays in deliveries left customers grumbling, with many turning to social media to voice their complaints. On Twitter, the hashtags #UPSfail and #FedExfail accompanied conversations about the delivery debacle.