I expect very large servers will be commercially operated and running bespoke software. When Tumblr adds ActivityPub support, it will be the largest Fediverse instance by far.
It will probably immediately cause those directly upstream to find some new bottlenecks in their implementation.
hmmm didn't know Tumblr was planning on doing that. Any more info on this?
They announced it, oddly enough on Twitter. Techcrunch reported it, among others. Flickr is also considering it.