There is no "thank you page", but: There is a page /confirm to confirm your mail.
So you can track the confirmation of user accounts, which is also nice. To do that: Log in to google analytics (you can import your conversions from there into adwords). Go to your administration/settings page, choose "edit" on the site you want to add the conversion.
There will be a paragraph saying "conversions" or "targets" (Not sure, i have no english "version"). Add a target with following configuration:
Keyword-Options: Match on HEAD
Then a conversion should be triggered every time a user confirms his mail.