I use both, but using both is a pain in the ass. I hate having to use two separate applications to update my status. I've added the Twitter application to my Facebook profile, but you have to click on my "boxes" tab to even see it. That's way too many clicks for Facebook users to see my Twitter updates. I want it on the front page of my Facebook profile, which it was, originally, before Facebook's crappy redesign. Also, I dislike having more than one status to update. The advantage of Twitter is that I don't need to begin each status update with "Michael..." The advantage of Facebook is the freedom from the 140 character limit.
There is a larger problem. Between Google, my RSS feeds, Twitter, Facebook, and my other email accounts, there are simply too many inboxes. Too many mediums, not enough content. Changes will be made.