Or do they forgo backup generators and count on shifting traffic to a new datacenter if there's a regional power outage?
I assume they do have backup generators, though I don’t know.
However if the sudden increase put that much load on the grid it could drop the frequency enough to blackout the entire neighborhood. That would be bad even if FB was able to keep running through it.
There are a lot of automated fail-safes on this, and apparently larger industry (which a datacenter is as well) will get disconnected from the grid automatically in emergency situations before they drop residential areas. But in the end they will drop one by one everything they need to keep the larger grid running. It's not even a networked "smart" management system, the distribution points automatically react to voltage and frequency drops and they're set up to break some things like industry earlier than others.
Short term they'd help but an instantaneous or unexpected massive traffic/CPU usage/user surge might pop too fast for the generators to start and kick in properly. Also, it might not be good for those big generators to start and stop over and over vs bringing infra back online in waves to limit spikes.
If the generators will help in an outage, why wouldn't they help in a brownout? You'd transition to generator when the voltage and/or frequency is outside of spec.
You'd typically have some short-term power protection to keep your datacenter running until the generators start.
I was skeptical about a datacenter that had less than 60 seconds of flywheel energy storage. But the data center manager said that if the generator doesn't start within 30 seconds, you're not going to get it started in an hour so having a huge battery stack that can power the datacenter for 15 minutes isn't going to help much.