That's recent though. I'm saying they scaled up quite a long way before doing anything like that. From the outside it looks like they didn't really branch out into things like this until 2008 or so.
No, even before the Newsfeed feature rollout of September 2006, they already had crazy complexity orchestrating multiple servers for many schools. They were only a "simple PHP+MySql" website when it was February 2004 with only Harvard students on it. Dustin said he was the one in charge of spinning up new servers for each new school and he said it was a nightmare of engineering effort.
But the point still remains, they had challenges to solve, but they were also simple, which made it possible TO solve those challenges.
They probably could have done most of America on a single server.
For example, even in 2009 Stack Overflow was running off two dedicated servers:
https://stackoverflow.blog/2009/01/12/new-stack-overflow-ser...