I use Twitter for different things than Facebook or LJ.

Twitter is more thought of the moment. Those things that you are thinking on the way home from work, or while you are exercising, that you forget about by the time you are making an LJ post.

Facebook is more for true daily/occasional status updates. Hey, this is how I feel today, or what I am doing today.

LJ is for the longer, more introspective stuff. The stuff that isn't just a passing thought that you want to share.