According to wikipedia,
The "like" button, which lets one user tell another that they like his or her content, and the "reblog" button, which easily reposts content from one tumblelog onto another, provide positive feedback that has given Tumblr an 85% retention rate, compared with 40% for Twitter.
Tumblr - Wikipedia, the free encyclopedia
Tumblr might be used by IMers in future. I'm not familiar with Tumblr but I have a feeling it might just creep into the top of the rankings. What do you think?