Nothing nefarious, just a simple cockup. This kind of thing happens ALL THE TIME in IT support.
BGP is a dynamic protocol. There is no single database of internet routes maintained by a central authority - each router tells the next router it is connected to about the networks it hosts and the networks it knows about. They don't necessarily know the final destination - just that traffic to network 10.11.12.13 goes to router x, and that router knows the next hop of the journey. This means each router doesn't need to know the details of how to reach every computer on the internet. Being dynamic, if a particular bit of cable gets cut, other routes dynamically kick in.
Incidentally, its possible to set up your router to announce to the internet that YOU own Facebook's networks as BGP is a very insecure system, designed in a more trusting age where there were no bad actors. If you can convince your peers (it helps if you are a trusted ISP) you can hijack the traffic intended for Facebook and send it to your own network instead. This has happened a few times in the past, usually accidentally when someone makes a typo in their router configuration. That's what some people initially thought mmight have happened here.
However, if I am the network guy at Facebook, and I accidentally make a change to BGP that effectively says "stop telling the internet about all of Facebook's networks', all the routers connected to Facebook will dutifully delete all the routes that lead to Facebook. Nobody will be able to get there. The system did what you told it to do. That's essentially what happened here.
In a simple network where I have one building where I work, my computer is connected to the internal network, and the router is in the same internal network, such a mistake is annoying but not catastrophic. I can simply log back into the router via the local network, reverse the change I made, and then its fixed, probably in minutes.
In a big company like Facebook, I might not even be in the same country as the router I just broke. I might be working from home, relying on a remote connection into Facebook's network which is now gone since I disconnected all the networks. Once the change is made, there might be no way for my computer to reach the router to fix it.
The outage as described however appears to be the fault of bad design.
If you can afford it, and I think Facebook really can, you should always have an out of band method to access critical infrastructure remotely if needed for this exact situation. Out of band means that it doesn't rely on the device working correctly to get to the device.
You can buy such systems off the shelf easily enough.
Hell, an old laptop in the datacentre with a simple 4G network adapter and serial cables attached to the console ports of the core routers is perfectly doable as an emergency access system - I've done this myself in the past.
Now, with a network as complicated as Facebooks, maybe they did have out-of-band management, but it took them a long time to figure out how they broke it and how to fix it.
Or maybe their 'emergency system' only worked from the office but due to COVID everyone was at home and that risk was never realized until now.
I hope this will result in some much-needed better design at Facebook. Not going to hold my breath though.