"We're aware people are having trouble accessing our services. We are working on this now," Meta spokesman Andy Stone said in a post on X.
At about 12:00 US east coast time (1700 GMT), the sites seemed to be returning to normal, about two hours after the outage reports first emerged.
According to the DownDetector website, reports that Facebook was down peaked at around 500,000 at 1530 GMT. Instagram peaked at about 70,000 reports at the same time.
Threads, the rival to Twitter that was launched in 2023, also suffered reports of outages, though WhatsApp, Meta's messaging service, seemed spared.
Facebook suffered a similar outage in October 2021 which was attributed to technical issues, not a security hack as originally feared.
At the height of the problem on Tuesday, Facebook's status page, intended for advertisers, said the site was suffering "major disruptions" and that "engineering teams are actively looking to resolve the issue as quickly as possible."
Users trying to access Facebook were asked to log in but were unable to sign in using the correct password.
On Instagram, mobile users were seeing their feeds not refreshed.
Reports said that Meta's virtual reality headsets were also suffering problems, with the device's Horizon World platform not allowing users to sign in.
Facebook is the world's largest social media platform, with three billion active monthly users.
Instagram has about 1.35 billion users, according to the latest data.
X, formerly Twitter, saw a spike in online activity as users were locked out of the Meta sites.
"Testing, testing... affirmative, everything is functioning smoothly here," wrote X CEO Linda Yaccarino, taking a dig at its rival.
US media focused on the fact that the outage took place on Super Tuesday, the day that millions of people were voting in primaries in 15 states and one territory.
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