Well it was more the interest graph. I follow people on twitter for whom I have no idea what their email address is. So finding all the people I follow on Tumblr is a simple matter of getting my follow list and matching up with registered Twitter handles.
And yes, Tumblr can do their own service the Tumblr version of Twitter, but then users are 'meh' because really they want just one thing to follow/monitor not a dozen (the Google+ problem in spades).
So what Tumblr really wants is a replacement for Twitter that has the same attributes, but creating that replacement is a made harder by the whole 'one place to go problem.'
One possible strategy is to empower a 'neutral' third party. As more and more people are alienated by Twitter, even competitors of Tumblr can see the benefit of empowering this neutral third party and so by not 'owning' the service they empower the service to be successful. I realize that is a bit zen but its really the only way this works as far as I can tell.
And yes, Tumblr can do their own service the Tumblr version of Twitter, but then users are 'meh' because really they want just one thing to follow/monitor not a dozen (the Google+ problem in spades).
So what Tumblr really wants is a replacement for Twitter that has the same attributes, but creating that replacement is a made harder by the whole 'one place to go problem.'
One possible strategy is to empower a 'neutral' third party. As more and more people are alienated by Twitter, even competitors of Tumblr can see the benefit of empowering this neutral third party and so by not 'owning' the service they empower the service to be successful. I realize that is a bit zen but its really the only way this works as far as I can tell.