Social media platforms Facebook, Instagram and WhatsApp, and Facebook Messenger, were hit by a three-hour outage.
A site that monitors site outage, Downdetector.com, shows Facebook had been down since Sunday evening in much of the world.
Thousands of people reported outages concentrated in the northeastern US, Europe and the Philippines.
It appeared to be back running several hours later.
A Facebook spokesman declined to give a reason for the outage, but said the issue has since been resolved and apologised for any inconvenience.
#FacebookDown, #instagramdown and #whatsappdown were all trending on Twitter globally.
Facebook acquired Instagram in 2012 and WhatsApp in 2014.
There are more than 1.52 billion daily active Facebook users, according to the social media network's website.
