Ah.. sorry about that. That pop-up is loaded straight through an AJAX call to JomSocial, and bypasses the system plugins. I don't have a JomSocial installation to test against right now (weekend), but if you want to try the following, it will hopefully work. If not, I can try to test early next week and come up with something.
Instead of the JFBCLogin tag, use the following, which will get the correct Facebook Login Button code:
require_once(JPATH_ROOT . DS . 'components' . DS . 'com_jfbconnect' . DS . 'libraries' . DS . 'facebook.php');
$jfbcLibrary = JFBConnectFacebookLibrary::getInstance();
echo $jfbcLibrary->getLoginButton();
You'll need to put the php open tag at the beginning of that code and a closing php tag at the end if it's in a PHP section. Hope that makes sense.
Hopefully that will work for you, but again, if not, let us know. If it doesn't, please also let us know what version of JomSocial you're using.
Thanks,
Alex