Facebook and its family of apps, including Instagram and Whatsapp, experienced the most severe outage in their history.
Key services were unusable for users globally for much of Wednesday. Facebook users reported not being able to post, while those on Instagram were not able to refresh feeds or post new material.
Facebook Messenger's desktop version did not load, while users on the mobile version reported glitches. WhatsApp, Facebook's other messaging app, had similar problems.
The last time Facebook had a disruption of this magnitude was in 2008, when the site had 150m users - compared with around 2.3bn monthly users today.
"We're working to resolve the issue as soon as possible," Facebook said in a statement, but did not reveal the cause of the interruption.
The tech giant, which gets most of its revenue from advertising, told Bloomberg that it was still investigating the overall impact of the outage, “including the possibility of refunds for advertisers.”
The company denied rumours that the outages were not a result of a Distributed Denial of Service (DDoS) attack, which involves flooding a target service with extremely high volumes of traffic.
An NBC News broadcaster reported the cause was related to "overloaded" databases.
The problems appeared to have been largely resolved by Thursday morning.