Corporations don't do anything unless there is a executive sponsor and business need/attached revenue. Probably they have never needed a maintenance mode, aka self imposed downtime. The only thing worse that unexpected downtime is some manager causing the need to turn on maintenance mode. They would lose their job.
We had maintenance mode at MySpace. We could shutdown any part of the site with feature flags that can be turned on for ranges of users. Very useful for bringing back the site after an outage and allow the caches to fill without overloading the underlying dbs. I am sure twitter has the same, they had scalability issues at the beginning . I guarantee they have a mode to disable posts and mode to disable authentication so they can recover the underlying systems .
