I think I'm going to tell a girl I'm seeing I didn't like her being on twitter.
Why don't you want the girl that you're seeing to be on Twitter?
She talks to other people on Twitter.
You are infringing on another person's rights by telling them you do not want them to be on social media. You are being controlling and it is not your place to tell anyone what they can do online.
She has to do what I say.
She doesn't have to do anything that she doesn't want to do, because she's an adult. Trying to tell her that she can't go on Twitter, or do anything else, is really controlling and abusive. I think you should stop this kind of behavior.