system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does facebook update itself, yet i've never seen it go down for maintenance?
gpt
Each time you visit facebook, you're hitting one of their hundreds (if not thousands) of servers.

Knowing that, they're probably doing rolling updates across their entire server farm.

For example, if facebook is served by 1000 servers, they might bring down 250 of them for updating, while keeping the remaining 750 up to handle visitors.  Once the original 250 are updated and running again, they'll bring down another 250, so on and so forth.

So at no time are all of their servers down for maintenance.