I love keeping a journal and have done so for a fair few years quite diligently and before that, on and off. I have friends who have journals from their grandparents and some even further back.
The one condition I have, though, is that I don't share it with anyone except someone I explicitly want to share it with. With everything being absolutely private and in my control to carry with me, by default, it would be killer. But this could well be opposite to Facebook's own direction.
All-in-all, this is a rehash of the news feed with a few more controls. But the presentation is really nice.
I can see how the practice started; help spread the word. It just doesn't inspire confidence in me for a site where I'm going to end up posting all sorts of details of my life...
Still, I'll be interested to check it out myself once I can.