It's not just tumblr or posterous, but all the new web 2.5 blogging apparati out there have found a middle ground between blogging and tweeting. The blog is now meant to roll over. It doesn't really matter what was there a week ago, except to award cred, when a prediction was awfully right, or to denounce, when predictions went beyond sour.
It's not the fault of these services, they're just trying to keep up with the pace. It's like archives are just taking up space (both screen, and disk) the rate of new news makes anything obsolete by this afternoon.
This is great and all, but there's still a big problem out there. Fast web publishing wasn't only about getting things out on the web quickly, it was about allowing people to express their ideas easily.
Tumblr and posterous and all the others are allowing for a deeper conversation than twitter, but it's still a conversation that's moving through time very quickly.
Blogs have become worse as a source of reference material, and there is no good alternative.
Next time perhaps I'll speak about why Wikis don't fill that particular void very well.