I didn't realize you were the OP in the other comment. I would maybe leave twitter if there was a more attractive alternative and I didn't have an attachment to their brand or my content.
I'm not sure in the case of Twitter what could help. Other than the "sorry to see you go, we accept/appreciate your feedback in the textbox below".
It is a hard question of what makes an user leave. Why do we stop using tools, visiting places?
From HN I've read the recommendation to listen to active users (after all, you can get more data from them than one who left).