"We're aware that some people are having trouble accessing our apps and products," Facebook said on Twitter. "We're working to get things back to normal as quickly as possible, and we apologize for any inconvenience."
Outage tracking site Down Detector logged tens of thousands of reports for each of the services. Facebook's own site would not load at all for about an hour on Monday; Instagram and WhatsApp were accessible, but could not load new content or send messages.
The reason for the outage was not immediately clear. However, multiple security experts quickly pointed to a Domain Name System (DNS) problem as a possible culprit. Around 1 pm ET, Cisco's internet analysis division ThousandEyes said on Twitter that its tests indicate the outage is due to an ongoing DNS failure. The DNS translates website names into IP addresses that can be read by a computer. It's often called the "phonebook of the internet."
More than four hours after the outage started, Facebook CTO Mark Schroepfer tweeted: "We are experiencing networking issues and teams are working as fast as possible to debug and restore as fast as possible."
A spokesperson for the company did not respond to a request for comment from CNN Business.
"I don't know If I've seen an outage like this before from a major internet firm," said Doug Madory, director of internet analysis at network monitoring firm Kentik.
For a lot of people, Madory told CNN, "Facebook is the internet to them."
Firms sometimes lose internet connectivity when they update their network configurations, Madory said. That's what happened in June to Fastly, a US cloud computing firm, which experienced a global internet outage for about 50 minutes.
But the fact that a company of Facebook's size and resources has been offline for over three hours suggests there is no quick fix for the issue.
Roland Dobbins, principal engineer at digital security firm Netscout, said Facebook will likely work to gradually restore service, and that it could take some time for routed information "to be received and propagated worldwide."
The outage came the morning after "60 Minutes" aired a segment in which Facebook whistleblower Frances Haugen claimed the company is aware of how its platforms are used to spread hate, violence and misinformation, and that Facebook has tried to hide that evidence. Facebook has pushed back on those claims.
The interview followed weeks of reporting about and criticism of Facebook after Haugen released thousands of pages of internal documents to regulators and the Wall Street Journal. Haugen is set to testify before a Senate subcommittee on Tuesday.
Shares of Facebook were down more than 5% in midday trading Monday, putting it on pace for its worst trading day in nearly a year.
Story first published in CNN