Go ahead and read Matt Haughey's post on why he left Twitter. But I wanted to pull out this bit:
I didn’t like that everything I wrote ended up being hard to find or reference, and even hard for me to pull up myself when I wanted, where a blog makes it pretty dang easy to see everything you wrote about in the past.
If I'm analyzing my reasons for blogging and/or microblogging on my own domain this is likely #1. I love having a history of my thoughts, guesses, observations, and photos. And I love that I own it.