With certain configurations using varnish the Facebook Connect quick registration fails due to its usage of _GET for logging the user in:

An approach to change quick registration to work with _POST is:

a) Change form_state['redirect'] = 'fbconnect/register/create' to use menu_set_active_item, menu_execute_active item instead (when quick registration

b) Have fbconnect/register/prompt POST directly to fbconnect/register/create via JS usage in case quick registration is enabled.

c) Have fbconnect/register/create that is selected via _GET display a small JS that does a POST to the same page instead.

This also is combined with the HTTP paradigm that POST, PUT, etc. requests should be used for changing data and GET requests only for display purposes.

Thanks and best Wishes,

Fabian