I have Delicious, where I collect links. And these links are posted to Livejournal once a day using a handy (slightly hacked) script, which I keep meaning to rewrite. These also go to andyduckerlinks - but comments on there only get checked once a day, and there's not much conversation there. Handy if you only want to read my links and nothing else :->
I have Facebook, where I occasionally update my status (and also do event stuff)
I have Twitter, which takes a feed from Delicious and a feed from Facebook. If you are reading me here and on Facebook then Twitter isn't going to add a lot of value for you, except as an aggregator. I do occasionally reply to things people have said on there though, but Twitter isn't a great place to hold discussions IMHO - it's declarative, not discursive. I'm mostly on there so I can read the links that other people post there while I'm waiting for the bus.