Make this code change and you should be good.
In administrator/components/com_jfbconnect/assets/facebook-api/base_facebook.php, at line 1477, replace
$query = '?' . implode($retained_params, '&');
with
$query = '?' . implode('&', $retained_params);
In PHP 8.0 and later, the order of the arguments for the implode statement is strict and requires the glue to be first. Prior to that, the parameters could be passed in either order. You're running 8.0.12 now...
Checking this fix in for the next bug fix release.
-Melissa