The cause of the outages on the platforms, all owned by Facebook, was not immediately clear.
But error messages suggest there's a problem with Domain Name System (DNS), which allows web addresses to take users to their destinations.
In Australia, outages began being reported by users of Facebook, Facebook Messenger, Instagram, WhatsApp from around 3am AEDT local time on Tuesday.
Thousands of users reported issues accessing Facebook websites and aaps, according to Downdetector's Australian site.
A similar outage at cloud company Akamai Technologies Inc took down multiple websites in July.
Facebook's internal systems used by employees also went down, the Associated Press reported.
Security experts tracking the event said the outage could have been triggered by a configuration error, which could be the result of an internal mistake, although sabotage could not be ruled out.
An outside hack was viewed as less likely.
A massive denial-of-service attack that could overwhelm one of the world's most popular sites would require either coordination among powerful criminal groups or a very innovative technique.
Facebook acknowledged users were having trouble accessing its apps but did not provide any specifics about the nature of the problem or how many were affected by the outage.
"We're working to get things back to normal as quickly as possible and we apologise for any inconvenience," Facebook said.
Shares of Facebook, which has nearly two billion daily active users, fell 5.5 per cent in Monday afternoon trading in the US, inching towards its worst day in nearly a year.
Downdetector - which only tracks outages by collating status reports from a series of sources, including user-submitted errors on its platform - showed there were more than 50,000 incidents of people reporting issues with Facebook and Instagram.
The outage might be affecting a larger number of users.
Meanwhile, the social-media giant's instant messaging platform WhatsApp was also down for more than 35,000 users while Messenger was down for nearly 9800 users.
Facebook has experienced similar widespread outages with its suite of apps this year in March and July.
Several users using their Facebook credentials to log in to third-party apps such as Pokemon Go and Match Masters were also facing issues.
"If your game isn't running as usual please note that there's been an issue with Facebook login servers and the moment this gets fixed all will be back to normal," puzzle game app Match Masters said on its Twitter account.
The outage comes a day after a Facebook whistleblower accused the firm of repeatedly prioritising profit over clamping down on hate speech and misinformation, and said her lawyers have filed at least eight complaints with the US Securities and Exchange Commission.
© AP 2021