Facebook on Friday said some users were again having problems accessing its services, just days after a massive outage.
"We're aware that some people and businesses are having trouble accessing Facebook products," a spokesperson told AFP.
"We're working to get things back to normal as quickly as possible and we apologize for any inconvenience."
Facebook did not provide details on the cause or scope of the problems, which caused users to flock to Twitter to voice frustration.
"What's up with Instagram?" read a tweet that included a picture of cartoon character Bart Simpson sitting in a corner in apparent punishment.
"It's not even 4 days and it's already down again."
Website trouble tracker DownDetector showed spikes in reports of problems accessing or using Facebook and its photo-centric Instagram network as well as Messenger and WhatsApp.
"Problems with Instagram, Facebook, Facebook Messenger and WhatsApp AGAIN!" read a lament in a DownDetector chat forum.
Hundreds of millions of people were unable to access Facebook, Instagram or WhatsApp for more than six hours on Monday, underscoring the world's reliance on platforms owned by the Silicon Valley giant.
In an apologetic blog post, Santosh Janardhan, Facebook's vice president of infrastructure, said that day's outage was caused by "configuration changes" on routers that coordinate network traffic between data centers.