"X is Following your Blog" via Jetpack?

There are different things you could do to improve things:

1. You can purchase the Site Redirect upgrade to automatically redirect folks accessing your old site to the new one.
2. You can make that old site private so no one can subscribe to it anymore.
3. You can move your subscribers from your WordPress.com site to your Jetpack instance by following the instructions here:
   https://jetpack.me/support/subscription-migration-tool/
4. You can then access your Jetpack Subscribers list by following these instructions, export your subscribers list thanks to the link at the bottom of the page, and import them into Mailchimp.

I hope this helps.

Thanks