What he means is this: you subscribe by telling the site your email. Then, if they ever find your email in one of the publicly released documents, then they will notify you by email.

What if your email is the account being compromised?

I guess thats why you might add a mobile number

Huh? You don't need to give them anything except for your email address. They only look for your email address in the compromised databases.

Yes you do if you subscribe for notifications.

I still don't understand. Why?

The OP comment was: "It'd be cool if they added an option to subscribe for $10/year for a quick SMS and email notification if your account is compromised. I'd get it for myself and my family."

To enable this - you are giving your email, password, Payment details and Cell Phone number. The site as it stands today doesn't ask for any of this -- but if they were to take payments and do SMS notification they would.

If we decided to do notifications I would expect users to not re-use passwords from other sites. I would also expect that such a service would require a trusted security brand behind it to work.