From what I am finding, it fits into the gap between blogging and micro blogging. I will admit, I used to host my blog on Posterous, but have changed to WordPress (again!) due to the limitations with SEO and all that kind of thing. I mean, some people have had great success with it, I had some good success with it, but I am back with WordPress now for the sake of customization. I do have a use for Posterous shortly, and it should hopefully demonstrate to others a good use for a platform that sits between WordPress and Twitter.