Euronews has reached out to Facebook for comment but did not receive a reply at the time of publication.
A third-party outage map suggested the problem was global - DownDetector monitors posts on other social networks for users mentioned a loss of service elsewhere.
Facebook did not say why users were experiencing issues, but The Independent reported that some users were receiving messages that said the site was "down for required maintenance", indicating the outage could have been scheduled.
On Twitter, the term #Facebookdown trending for much of the day, with users lamenting that they could not post anything or even log into the website.
The platforms weren't down for everyone, but many people were unable to access their profiles, with glitches in how the sites worked.
"You make it sound as if it's the end of the world just because you can't be on Facebook".
Some media outlets branded the outage as the biggest in Facebook's history.
A "DDoS" attack refers to a "denial of service" cyber-attack in which an individual or entity would overwhelm servers to block the public from accessing a certain page or network.
On its developer site, Facebook said: "We are now experiencing issues that may cause some API requests to take longer or fail unexpectedly".