Why? How is that better than using WordPress and RSS?
If you host your own Wordpress install, the ad dollars go to you.
If you're Mark Zuckerberg, bloggers blogging on Facebook is better than bloggers blogging elsewhere. That's the circumstance under which blogging on Facebook is better.
I'd certainly run my own Wordpress, but not everybody wants to.
I suppose I just connect more so with a facebook profile than with a traditional blog.