“The underlying problem is that Tumblr is not easy to use.”
Tumblr plans to make changes to appeal to new users and improve growth. The post states that Tumblr is currently difficult for new users to use and curates content poorly. Tumblr aims to improve how users discover and sign up, encourage frequent engagement, and boost creators' ability to attract audiences. Specific changes mentioned include updating the confusing reply and reblog system to make conversations clearer and removing duplicate reblogs. Interestingly, the post does not discuss Tumblr's plans to integrate with the ActivityPub protocol. Tumblr has had a turbulent history but has stabilized under its current ownership, though some long-time users may not want a more optimized social feed.
There's a soft spot in my heart for Tumblr. I hope they find their way and become a bigger player.
Tumblr's problem, at least when I stopped using it 10 years ago, was that it was kind of an SJW hellscape. The most toxic of toxic activists made their home there, and it was a profoundly unpleasant Internet experience. They'd have to pay me to use their site again.
By that same logic reddit was a harbor of alt-right and other shitty people but if you curated your experience with it you could mostly avoid it. Similarly so with tumblr in my experience.
Tumblr didn't have a ton of curation tools back when I was a regular user, but Twitter did, so I ended up spending most of my time online there until October of last year. The incident where Elon Musk carried a sink to the office was all the info I needed to know Twitter was about to take a nosedive. Luckily I really enjoy the fediverse.
You can search through tags to find people but honestly i find reblog chains suitable for finding people i'm interested in after getting started with some fandom tags and other stuff.