This paper investigates how people express social identity at a large scale on a social network. We looked at communities of users on the Twitter website, and tested two established social-psychology theories that are usually performed at local scale. We found evidence of Communication Accommodation Theory, where community members vary their language characteristics depending on which community they are communicating with. We also found the level of linguistic variation correlated with how isolated a community was: evidence that there is Convergence between linked members. This demonstrates the power of methods which analyse subtle human behaviour on social networks.