Facebook users around the world on Monday reported the social network was inaccessible, with many taking to Twitter to voice their frustration.
Reports of issues on the downdetector.com website began around 2100 GMT, with users particularly affected in North America and Europe.
Some users attempting to log in to the site were met with a "Service Unavailable" message, while others received a longer note that said it would return online soon.
"Facebook is down for required maintenance right now, but you should be able to get back on within a few minutes," the longer message said.
"Thanks for your patience as we improve the site."