Just signed up for Ex.fm at the suggestion of a Twitter buddy, when I noticed an interesting option in the “Find Friends” step- Tumblr friends. You can use your Tumblr social graph! Typically a “find friends” step includes Facebook, Twitter, and email (Gmail) contacts to bootstrap your friends in a new site.
Hopefully this Tumblr option will appear across the web. I would love to see more new social sites enabling access to our Tumblr (and Instagram?) social graph. It’s no secret Twitter is getting really unreliable for this. (They want apps to use Twitter authentication during sign-up onboarding, but don’t want them to let users find their Twitter friends. Why the heck would we auth with Twitter over Facebook, then?)
In any case- yay ex.fm, and yay for Tumblr for letting apps use this!