Actually Tumblr is more accurately defined as a <a href="http://en.wikipedia.org/wiki/Tumblelog">tumblelog</a>. One of the key reasons why I'm betting on the service is that they try to keep it simple but at the same time are active listeners of their community.

About the other platforms, perhaps the best way to go is to open the (now trendy) social graph, or instead use a service like <a href="http://www.hellotxt.com/">HelloTxt</a> to broadcast to them all.

I'm also quite interested to see what happens to Jaiku and see if Google manages to push their marketing into a Twitter killer and bind a myriad of technologies into one service (Gtalk + Notes + Bookmarks + Docs + gPhone + Jaiku).

On the trackback issue, i find it useful since we are creating a communication silo by broadcasting only within the social network.
The ability to trackback would function as a way to broaden the audience besides these closed applications.