(person who doesn't really know what they're talking about when it comes to services of this scale asking) Why does Tumblr need 1000 servers to run? You can serve a lot of text and relatively well compressed image files with not a lot of hardware.
1000 servers actually sounds surprisingly small. It was a fair bit higher than that a decade ago, and that was on bare metal then, vs (afaik) cloud VMs now. Then again, their traffic is a lot lower now, but still nothing to sneeze at.
That's without getting into video content.