The thing about Facebook and Twitter is that brevity is of the essence. So you just shoot from the hip and post without a second thought because who reads past the first 2-3 lines anyway? And if you change your mind about the post, then you can delete it as easily as you'd posted it.
(Yes, everything stays on the internet, but at least it doesn't appear on your timeline anymore.)
I was just wondering why it's easier to tweet or post a Facebook update than to actually write a blog. Duh.
Of course, this overlooks the people who tweet images of text to get around the 140-character limit and who write blogs in their Facebook updates anyway. So it's really just down to user laziness, isn't it?