so when you sign up for tumblr, you get to choose a url and it shows you

"<yourchoice>.tumblr.com"

when you write it.

I had an idea that you could have a join-federated-tumblr.org page, where it asks you to choose both a blog name AND the domain it's on, and you finish up sign up on *this* page, instead of the instance page.

This will help new comers to actually use federation and sign up for different instances