Twitter is a weird social beast in of itself. Obviously, you can use it to just simply promote when you upload a new video. But you can also use it as a way to interact with certain special groups or topics.
Have a video where you go to a Renaissance Fest? Post it to the specific ren fest twitter. Reviewed a game? Send it to the game developer.
Part of it is also just putting in the time to be social on there as well. Doing polls, posting photos, interacting with other twitter users, etc. Again, because of that, it's a completely different animal, meaning you'll have to be building up your own following on there in addition to your following on YouTube.