My guess is not very long. I've been on a project with three people where if we all quit, it would run until AWS EKS switched off the version of Kubernetes we were using; I'd expect from experience at FB that there's way more done to keep plates spinning on the Twitter fleet and within a couple of days there would be a SEV that no-one was around to respond to.

Of course they'd actually be pulled down by law enforcement much quicker than that if all their content mods left.