Not certain how willing I'd be to send a third party the unhashed email address of every (or even just a few) signups. Seems to be a big privacy violation and the users of the sites that use this should clearly state what they are doing.

I know tracking companies were getting in a lot of trouble for just having hashed versions of emails sent out and a lot of users of those sites were very mad. I'd be pissed if my email was being sent along without my knowledge.

EDIT: Outside the privacy risk though this actually looks really neat and helpful. I'm assuming this is using the rapportive API or something very similar. Neat service, just too many privacy risks for me to use it.