A blog post has lasting value.
A tweet stream is more ephemeral, it can evaporate almost instantly.
Not saying that quick comments shouldn't be tweeted, but when you figure something out that's not trivial, if it has value, you're wasting it if you put it on Twitter or Facebook.
The mission of blogging is to empower all of us to go directly to each other with our expertise. So if you know something as well as anyone else, or you learn something or know something that should be shared, then you should share it on your blog.
Blogging needs your help. There's cobwebs in the blogosphere. I want to dust everything off, and start linking our stuff together, and get some developer energy flowing, and let's do some new stuff.
Blogging is a good idea. Most people aren't compelled to share their ideas, thinking, knowledge and expertise. But if you're one of them, please use the tools, or if they fall into disuse, we'll just have to invent them again. Let's keep it growing.