Facebook, WhatsApp and Instagram – which all run on shared infrastructure as Facebook is the parent company, has not worked 7PM Greece time.
Facebook Messenger and Workplace have also stopped working.
At time of writing, it is unclear what has caused the issue although the company has suffered outages before.
Data from the website suggests the outage is affecting user accounts across the world.
Some people also reported problems using Facebook's virtual reality headset platform, Oculus.
And apps which require Facebook logins were affected, including Pokémon Go.
Facebook said: "We are aware some people are having trouble accessing our apps and products. We are working to get things back to normal as quickly as possible and apologise for any inconvenience."
It is rare for such problems to affect a single tech giant such as Facebook, or for the issue to remain unfixed for a long time.
Several other tech companies, including Reddit and Twitter, have poked fun at the social media giant's predicament - prompting responses from the affected apps.
hello literally everyone
— Twitter (@Twitter) October 4, 2021
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