Tumblr is old. It is part of the foundational history of the social Internet. Together with Livejournal and Blogger, it was the baseline others iterated on. Like Flickr (more in a second), it lost out some, changed hands a few times, and struggled to stay relevant. But it remained, partially scrapping by on nostalgia, partially on enthusiast's hopes visions of what it could be and become.

Tumblr is huge. By joining the Fediverse it would instantly become the largest player in the space, adding 13 million actors to the network. And yes, it is a siloized instance, but one could argue that that's not unlike the other monster instances out there.

Tumblr's current design would push advanced Fediverse functionality into the federation. In fact, I'd probably use it over other implementations.

Being able to follow, interact with, and redistribute Tumblr content into the fediverse would massively improve diversity of thought, opinion, and user base. The fediverse currently essentially consists of former Twitter users wanting to have Twitter without Musk and nerdy oldtimers like me who wanted anything but Twitter, which is why we left or never joined it long before Musk was a thing.

Tumblr's success in the space would encourage others to do the same, open the walls, become federated, creating a rising tide that lifts all boats, and gives the Silos a run for their ill-earned advertising money. This isn't just about the current Mastodon-rush. Imagine being able to post your videos to a PeerTube instance and having instantaneous access to 22 million potential subscribers. YouTube might have to make some pretty drastic changes to stay relevant.

In concert with Flickr also thinking about federating, a Quora clone being developed, and Friendica being a better Facebook than Facebook ever was, the push from a long tail into a top wave movement of distributed servers and services the one big fear, that of services dying and leaving a vacuum, would lessen considerably.