I have an account with all three, but never really used MySpace - only signed up to see something a friend had posted.
I use Facebook all the time! I've reconnected with some old friends & frankly, am addicted to some of their games LOL
I do have a Twitter account, but don't check it very often. I am "following" a few people & find it interesting. I just started a Twitter account for my guild & thought I could post about our upcoming speakers & workshops. We'll see what happens.