Facebook said on Tuesday users were having trouble accessing the social network and its other applications such as Instagram, but did not explain the cause of the outages.
It was the second instance in the past two weeks in which technical issues appeared to affect access to Facebook.
The independent monitoring site Downdetector showed a map indicating outages affecting most parts of the world, especially in North America, Europe and Asia.
The hashtag #FacebookDown was trending on Twitter.
"We know some people are having trouble accessing the Facebook family of apps. We're working to resolve the issue as soon as possible," Facebook said in a Twitter post at 1535 GMT.
Facebook gave no reason for the problems or details.
Facebook, which operates the Instagram photo-sharing platform and the WhatsApp and Messenger applications, has been under pressure in recent weeks over its handling of misinformation campaigns, and its hiring of a consultancy that sought to do damage control for the company.
Facebook has more than 2.2 billion active users worldwide.