Yes, I'm guessing not FullContact either. I'm guessing they're using PhantomJS or an automated browser to import the email address to their Google Contacts, and then "find a friend" on twitter, FB, etc with that email address. Slow, but they're delivering results by email, so customers can handle the delay.

If they're not doing this, this is a great (but hard) way to almost guarantee 100% accuracy.