facebook provides a mechanism to detect whether a connected user already has a local account, http://wiki.developers.facebook.com/index.php/Connect.registerUsers. This works only if the user provided the same email to both drupal and facebook, but even with that limitation is better than nothing.
More information: http://wiki.developers.facebook.com/index.php/Linking_Accounts_and_Findi....
Drupal for Facebook should use this mechanism on connect-enabled sites. And possibly on canvas pages sites, too.
Comments
Comment #1
neilnz commentedIt does? The fb_register module (in the contrib folder) does exactly that...
It also maps new users as they add the app.
Comment #2
Dave Cohen commentedYeah, I added fb_register, but never updated this issue. Thanks for reminding me.