Facebook wasn’t immediately available to comment on the reason for the outage. The problems appeared to be linked to a change the company made to networking instructions for how the world accesses its systems, according to outside experts.

Users began to receive error messages when trying to access Facebook platforms shortly before 12 p.m. ET, and the outage was “widespread and global in scale,” according to a spokesperson for the parent company of Downdetector, a site that monitors website outages.

The outage also caused widespread disruptions to Facebook’s internal communication tools, including some voice calls and work apps used for calendar appointments and other functions, according to people familiar with the matter.

The company told employees Monday morning that the cause of the outage was unknown and some staff were using Zoom to remain connected, the people said.

As the disruption continued through Monday afternoon, Facebook Chief Technology Officer

Mike Schroepfer

apologized to users on Twitter.

“*Sincere* apologies to everyone impacted by outages of Facebook powered services right now,” he wrote. “We are experiencing networking issues and teams are working as fast as possible to debug and restore as fast as possible.”

While even the biggest tech companies occasionally go offline unexpectedly, the duration and widespread nature of the disruption for Facebook and all its platforms is unusual, said

Tom Daly,

a networking expert who was previously co-founder of the internet company Dyn. “They have a massive infrastructure with a massive amount of complexity and they have to resolve all of that complexity to recover,” he said.

And while Facebook and Instagram are largely used for social reasons, WhatsApp is an essential communication tool for users around the world. The service has more than 2 billion active users.

The outage comes at a time of immense scrutiny for the social media giant. On Sunday, a whistleblower who provided documents that formed the foundation of The Wall Street Journal’s Facebook Files series went public.

Frances Haugen,

a former product manager at Facebook, said she acted to help prompt change at the social-media giant.

Facebook shares dropped more than 5% on Monday amid a broad-market selloff.

Users began reporting problems late Monday morning, according to Downdetector, a site that monitors website outages. A spokesperson at Downdetector’s parent company, Ookla, said the outage to Facebook and its other companies was “widespread and global in scale.”

Some Facebook services briefly went down in March, and workplace messaging platform Slack Technologies Inc. had a service disruption for several hours earlier this year. Last December, more than a dozen of

Alphabet Inc.’s

Google services, which include Gmail and YouTube, also suffered disruptions.

As many employees turned to cloud services and other connected platforms during the Covid-19 pandemic, the ripple effects of the outages have been felt more.

—Jeff Horwitz contributed to this article.

Write to Talal Ansari at Talal.Ansari@wsj.com and Robert McMillan at Robert.Mcmillan@wsj.com