I can only speak for myself. I personally user tumblr (only started few months ago) for private ramblings about random stuff that comes in my mind. My tumblr is blocked from search engine and I have not shared the link with anyone. I also blocked out comments and whatever other social media features there might be. I would have made it password protected if they allowed private tumblers to have access to API (you can't post to password protected pages from desktop publishing software AFAIK).
I know this is not how or why most people use tumblr. But thats ok. I use it only the way I like it.
I also have a "public" self hosted wordpress blog which I share with my friends and family and open to search engine.
posterous, to me, is just yet another blogging platform. Just like I don't feel like I need to try out every single new things comes out every week, I don't feel like posterous really sets them apart is such a way from all other existing blogging platform for me to even bother.