I am very active both on Twitter and on Facebook. When I post a new Tweet (from my phone or various Twitter clients I use), it is reproduced as my Facebook status and also appears in a sidebar in my online journal. When I post a new message on my journal, an announcement tweet is automatically sent, which also appears as my Facebook status. It sounds very complicated, but it's not. Install Alex King's Twitter Tools plugin for Wordpress and the Twitter app on Facebook, and you've reproduced my setup. I've always consumed Twitter activity in Twitter and Facebook activity in Facebook. I use their respective web clients as well as Tweetie for Mac OS X, Tweetie for iPhone, Facebook for iPhone, Facebook for Blackberry, Tiny Twitter for Blackberry, Twitteberry for Blackberry (2 twitter apps for Blackberry because I haven't found one that supports multiple logins).
If I see a tweet I want to reply to, I reply using Twitter. If I see something in Facebook I want to reply to, I reply using Facebook.
Luckily, I can't keep up with Facebook statuses because I seem to have accumulated over 800 friends over there; this effectively makes me not even try to keep up. The Facebook news feed provides an excellent way to "Hide" news from friends and companies I don't want to see daily information from, but it is still so overwhelming that I only really see the first page of updates when I go there for my fix.
My Twitter activity is still manageable because I only add people whose are compatible with me in Twitterland (no guilt over removing a good friend who tweets to the world every time he/she eats a bagel).
However, I do have a group of friends who, like me, duplicate tweets as Facebook statuses. I recently installed Seesmic Desktop, and for the first time, see Tweets and Facebook status updates roll in at the same time. I can reply to one or both right there in Seesmic, but the question is, in which community do I reply?
Seemic Desktop already de-dupes tweets coming in from multiple Twitter account communities. Hopefully, it will one day be able to group duplicate updates across Twitter and Facebook accounts and reply to both at the same time.
In any case, it's nice to see attempts to harvest updates from multiple communities to provide a single place to participate online. I haven't seen any application do it right yet, but I'm glad people are trying!