Verizon, T-Mobile & AT&T Customers Report Outages after Facebook Goes Down
The cause of the problem is not yet known

Facebook, Instagram, and WhatsApp have suffered outages as of Monday afternoon, which is still ongoing.
Facebook wrote in a brief statement:
“We’re aware that some people are having trouble accessing our apps and products.
“We’re working to get things back to normal as quickly as possible, and we apologize for any inconvenience.”
We’re aware that some people are having trouble accessing Facebook app. We’re working to get things back to normal as quickly as possible, and we apologize for any inconvenience.
— Facebook App (@facebookapp) October 4, 2021

The cause of the problem is not yet known.
Meanwhile, users of wireless carriers Verizon, T-Mobile, and AT&T also suffered outages.
It is not clear if those outages were Facebook-related.
According to The Guardian: Facebook issued a statement on Tuesday confirming that the cause of the outage was a configuration change to the backbone routers that coordinate network traffic between the company’s data centres, which had a cascading effect, bringing all Facebook services to a halt.

Something else that was interesting as per Disclose.tv:
"DNS records that tell systems around the world how to find http://Facebook.com or http://Instagram.com got withdrawn from the global Internet routing tables."
UPDATE - DNS records that tell systems around the world how to find https://t.co/K2pmpa1kSP or https://t.co/Wma7z2wdXc got withdrawn from the global Internet routing tables.
— Disclose.tv (@disclosetv) October 4, 2021
The Independent reported:
Facebook appears to have had its DNS records taken from the global routing tables. That’s according to Brian Krebs, a cyber security expert who runs a popular blog.
In slightly less nerdy speak, that means that effectively Facebook.com, Instagram.com, and presumably the rest have had their records wiped from the internet’s address book. When you type one of those URLs into your internet browser, it should be able to speak to Facebook and ask it where it needs to go – but the system that does so has been withdrawn.
It’s like turning up at the Facebook office for a meeting, but the receptionist isn’t there. You (or your computer) are just stuck at the desk, since you (or it) don’t know the number of the office door you’re trying to get to. (Or something like that analogy.)
It’s not clear why that happened. Facebook is so big that it runs its own DNS – unlike other, smaller companies – so only someone at Facebook would have the power to stop it running, too.
Confirmed: The DNS records that tell systems how to find https://t.co/qHzVq2Mr4E or https://t.co/JoIPxXI9GI got withdrawn this morning from the global routing tables. Can you imagine working at FB right now, when your email no longer works & all your internal FB-based tools fail?
— briankrebs (@briankrebs) October 4, 2021
You can monitor the outage maps from DownDetector by clicking here.