Click the gear symbol at the top right of any Facebook page, and choose "Account settings" from the drop-down list. Click "Notifications" in the left menu. On the Notifications Settings page, choose "Email" and then click "Edit."
Check "All notifications, except the ones you unsubscribe from" under "You'll receive."
Click "See All" followed by "Turn On" against the types of comments you want to receive emails about, such as "Comments on stories on your timeline," for example.
Respond to a comment when you receive an email notification by choosing the "Reply" option in your email account. Type your response in the body of the email. When you send the email, your comment will appear on Facebook after the previous comment in the thread.