From a quick investigation, it looks like if you edit the /components/com_easysocial/themes/wireframe/registration/default.steps.php file, around the top, you'll see:
defined( '_JEXEC' ) or die( 'Unauthorized Access' );
?>
<?php if( $this->template->get( 'registration_progress' ) ){ ?>Update that to:
defined( '_JEXEC' ) or die( 'Unauthorized Access' );
?>
{JFBCLogin text=Login with}
<?php if( $this->template->get( 'registration_progress' ) ){ ?>Test that and let me know how it goes. Hopefully that's what you're looking for.
Thanks,
Alex