I can easily change email providers and remain in contact with the same people using the service and address they themselves choose. If I grow dissatisfied with these services, I can even go to the trouble of hosting my own email server.

I do not see a competitor to Twitter that is interoperable with their service. Nor is there a distributed option. Gmail and Hotmail are platforms that grew on top of an inherently distributed service. Twitter is a data silo.

And another: I can easily download my contacts and email history and take them with me. It's portable.