https://medium.com/@somospostpc/tumblr-is-tumbling-d6deb3bb8... Seems like this might have something to do with it.
Wow, that New Posts chart is brutal. This nearby link[1] seems to claim that Yahoo failed to "not screw it up", but the details are extremely vague. I'm unclear what could have caused such a dramatic reversal. ~Everyone left, but why? What changed? [1] https://qz.com/735583/
Young people hate writing very much text.
Young people hate using posting interfaces of the sort Tumblr utilizes (it still comes across as a CMS posting system).
Young people hate too much effort toward customization, which is required to make a Tumblr page look half decent.
Enter Snapchat, WhatsApp, Kik, Viber, Instagram etc.
Click, record video, send.
Click, snap, add four words, send.
No layout customization bullshit. No concerns about something on your page breaking by mistake and having no idea how to fix it to get it back to normal. No diary/journal or blog text aspect, which most people have zero interest in.
It's too useful for what its newer users want (the ones that would be signing up and providing growth).
Simply put, Tumblr is a dinosaur. It happened that fast.