There was no good way to do this before, but I just added the necessary hook to the trunk code, so here's how you can do it with that code.
First, you need to add the birthday field to the registration form. You'd do that like so:
add_filter('sfc_register_fields','add_birthday');
function add_birthday($fields) {
$fields[] = array('name'=>'birthday');
return $fields;
}
That adds the birthday field. You can also add 'view'=>'prefilled' to that array to make it only show up for people registering with FB credentials and not to people registering without them.
Next, you need to get the birthday information and save it somewhere when they submit the form. I added the sfc_register_request action to allow for that.
add_action('sfc_register_request','get_birthday');
function get_birthday($info) {
global $saved_birthday;
if (!empty($info['birthday'])) $saved_birthday = $info['birthday'];
}
Finally, you need to save the birthday with the user's account somehow. SFC's register function doesn't actually create users, it lets WordPress do that normally. So you can hook into the normal WP user_register hook to take the necessary action.
add_action('user_register','save_birthday');
function save_birthday($userid) {
global $saved_birthday;
if ($userid && !empty($saved_birthday) ) {
update_usermeta($userid, 'birthday', $saved_birthday);
}
}
Something similar to that will do the job.