Facebook was down for 40 minutes on Monday night (12.11.8), due to a "routine test".
The social network left many users in the US and other parts of the world unable to access their accounts for almost an hour, but didn't explain what they were testing.
A spokesperson for the company said yesterday: "Earlier today, a routine test caused users to have trouble accessing or posting to all Facebook services including WhatsApp and Instagram.
"We quickly investigated and restored access for everyone. We're sorry for the inconvenience."
People in Brazil, Peru and Mexico were among those affected.
They also had problems accessing Facebook-owned apps WhatsApp and Instagram.