Social network Facebook and its Instagram photo-sharing site became inaccessible around the world for almost an hour on Tuesday, the company later saying that an internal software glitch was to blame.
Millions of users were unable to access their accounts according to web and mobile accounts from around the world.
Other media reports indicated that the error that sent services offline was caused by Lizard Squad, the hack group associated with previous major denial-of-service incidents.
Facebook denied the claims in a statement despite Lizard Squad hinting on Twitter that it might have been responsible for it.
The hack group posted on its alleged account: “Facebook, Instagram, Tinder, AIM, Hipchat #offline #LizardSquad”.
“This was not the result of a third-party attack but instead occurred after we introduced a change that affected our configuration systems,” a Facebook spokesman said.
“We moved quickly to fix the problem and both services are back to 100 per cent for everyone.”
While Facebook and Instagram services were down on both desktop and mobile, users were greeted with the message: "Sorry, something went wrong. We're working on it and we'll get it fixed as soon as we can."
Dating app Tinder, which relies on Facebook to operate, was also affected by the technical glitch.
Some users took to Twitter to comment on the outage.
“While Facebook was down, I nailed a picture of my breakfast to a tree outside our house. Seven people have knocked to say they liked it,” wrote Alistair Coleman.
“Facebook and Instagram have been down for 30 minutes. Just read four books,” wrote comedian Chris D’Elia.
On Monday the official website of Malaysia Airlines was attacked by hackers claiming to be from the Lizard Squad, after also taking responsibility for Sony's PlayStation Network and Microsoft's Xbox Live outages on Christmas Day.