Social media sites Facebook and Instagram are available to users again after being down briefly.
The sites both appeared with error messages and users had been complaining on Twitter, while 40-minute outages from around 6am Irish time were also reported in the US and Asia.
A hacking group appeared to claim responsibility for taking down the two services.
However, Facebook and Instagram denied their services were hacked, blaming the outage on "a change that affected our configuration systems".
Facebook had 1.25 billion monthly active users at the end of September last year, while its photo-sharing service Instagram has 300 million active monthly users uploading around 70 million photos per day.
A hacking group called Lizard Squad tweeted "Facebook, Instagram, Tinder, AIM, Hipchat #offline #LizardSquad".
Yesterday it attacked the website of Malaysia Airlines, which was offline for at least seven hours, replaced by a message from the group.
Lizard Squad has claimed responsibility for several hacks over the past year, most of them aimed at gaming or media companies.
Last year it claimed it was behind attacks on Sony's online PlayStation network and Microsoft's Xbox site.