Yes. OnRegister is called just when they first register (mainly to pull in profile information). onAuthenticate is called anytime the login to the site, including that first time they register.
onAuthenticate should be used mainly if you want to deny certain users registration or add extra checks. onRegister or onLogin should be used if you want to do something after you've verified the user is really a Facebook user that's allowed to login to your site.
Hope that makes sense. We need to update our profile plugin guide a bit for some of the changes in the last few releases... so if something seems odd, just let us know!
Alex
Question on Reigster / Autentication Plugin
