Social network Facebook became inaccessible across much of the globe for a time on Tuesday, before returning to normal service.
Millions of users were unable to access their accounts.
Users in some countries also had difficulties accessing photo-sharing app Instagram.
Facebook said it believed its own engineers had caused the problem, downplaying claims that a hacking group had been responsible.
"Earlier this evening many people had trouble accessing Facebook and Instagram," a spokeswoman told the BBC.
"This was not the result of a third-party attack but instead occurred after we introduced a change that affected our configuration systems.
"We moved quickly to fix the problem, and both services are back to 100% for everyone."
The sites appeared to have been unavailable for about 40 minutes before coming back online.
Dating app Tinder, which relies on Facebook to provide its service, was also affected by the problem.
Some users took to Twitter to comment on the outage on Tuesday.
"I hope you all took advantage of the 35 second Facebook outage to Like a person in real life. #Faceboogeddon," wrote media analyst Arthur Goldstuck.
"While Facebook was down, I nailed a picture of my breakfast to a tree outside our house. Seven people have knocked to say they liked it," wrote Alistair Coleman.