[vinchuco]

Perhaps it would help to know what the product is.

Otherwise the only generic questions I can think of are: What made you leave? What change in our product would have convinced you to stay?

This is outside my area of expertise.

[sydneyliu]

For this example, let's pretend the product is Twitter. Since I think everybody knows enough about to Twitter to think about the scenario.

I worry that questions like "What could we change" would miss the cases of "I was just busy" or not hit the heart of the issue. Seems like it'd be really good question for a B2B product

[vinchuco]

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).