Facebook, Instagram and WhatsApp lit up again after a nearly six hour outage on Monday.
Facebook apologized and initially could not say what caused the failure.
But the company later said the problem was a faulty configuration change.
CEO Mark Zuckerberg posted on Facebook, quote:
"Sorry for the disruption today -- I know how much you rely on our services to stay connected with the people you care about."
The outage was the second blow in days to the social media giant on Sunday, a whistleblower accused the company of prioritizing profit over clamping down on hate speech and misinformation.
Shares fell nearly 5% for Facebook but rose half a a percent in after-hours trade after the platforms rebooted.
Several Facebook employees who declined to be named said that they believed that the outage was caused by an internal mistake in how internet traffic is routed to its systems.
Facebook lost about $545,000 in U.S. ad revenue per hour during the outage, according to ad measurement firm Standard Media Index.
Security experts said an unintentional mistake or sabotage by an insider were both plausible.