Facebook on Friday said users around the world again had problems accessing its services for hours due to a tweak of its system, just days after a massive outage caused in a similar fashion.

“Sincere apologies to anyone who wasn’t able to access our products in the last couple of hours,” a Facebook spokesperson told AFP about 21:30 GMT.

“We fixed the issue, and everything should be back to normal now.”

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 starting about three hours earlier.

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Facebook attributed the trouble to a configuration change at its computing platform and said that it affected users of the social network and  Instagram, Messenger and Workplace globally.

People flocked 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.”

“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.

Cyber experts think that problem boiled down to something called BGP, or Border Gateway Protocol — the system the internet uses to pick the quickest route to move packets of information around.

Sami Slim of data center company Telehouse compared BGP to “the internet equivalent of air traffic control.”

In the same way that air traffic controllers sometimes make changes to flight schedules, “Facebook did an update of these routes,” Slim said.

But this update contained a crucial error.

It’s not yet clear how or why, but Facebook’s routers essentially sent a message to the internet announcing that the company’s servers no longer existed.

The outage on Friday was not related to the one earlier in the week, according to Facebook.

Experts say Facebook’s technical infrastructure is unusually reliant on its own systems.

Social media outages are not uncommon: Instagram alone has experienced more than 80 in the past year in the United States, according to website builder ToolTester.

Facebook’s services are crucial for many businesses around the world, and Facebook accounts are also commonly used to log in to other websites.

Facebook’s apps are used my billions of people monthly, meaning outages can touch a large portion of the world’s population.