Each step along the way was basically just a small modification on the system before. AFAIK Jenkins doesn't come with the ability to safely deploy code to hundreds of servers out of the box, so building out the systems to make that the case would've been more work than just adding to what existed and for unknown benefit.

> And, how do/did you provision new servers? By hand, or did you use something like Chef/Puppet?

We use puppet for configuration management. That's been the case for most stuff since early 2011 for context. There's a lot more detail in our semi-recent infra/ops AMA: https://www.reddit.com/r/sysadmin/comments/57ien6/were_reddi...

On the other hand, since you weren't already using it for builds/running tests, it would have added some overhead.