Facebook users around the world have complained of being unable to access the social media site while "maintenance" was carried out.
Scores of people said they were experiencing problems logging into the website and app on Thursday evening.
People were also unable to access Instagram which is owned by Facebook.
The problems were fixed late on Thursday night but occurred again after lunchtime (UK time) on Friday.
A message displayed on Facebook when people tried to log in said: "Facebook will be back soon."
"Facebook is down for required maintenance right now, but you should be able to get back on within a few minutes."
It added: "Thanks for your patience as we improve the site."
A flurry of angry posts appeared on Twitter after Facebook and Instagram stopped working.
But the site was up and running for the majority of people by 10pm on Thursday night.
A Facebook spokesperson said: "Earlier today, a technical issue caused some people to have trouble connecting to Facebook and Instagram. We resolved this issue for everyone, and we apologise for any inconvenience."
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