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

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.