The migration isn't opt-in — all of your WordPress.com and email followers were transferred to the new blog. However, WordPress.com followers will only see updates in the WordPress.com Reader. If they also want to receive email updates, they'll need to sign up for those on your new blog. (They won't receive a notification about it, but you're welcome to invite your followers to sign up for email notifications if they want.)
The remaining followers that you see on your WordPress.com site are social media followers who get your updates via Publicize. If you'd like those followers to keep getting updates from your new blog, you're welcome to set up Publicize on the new blog: http://jetpack.me/support/publicize/
Please let me know if you have any questions about that!