This is definitely an option we'll be adding in the future. Right now, JomSocial is actually telling JFBConnect to insert those buttons, and we don't have as much control over how they look as you might think since we're not the ones actually doing it.
With that said, you should be able to make a minor code edit to use the icon by default instead of icon+text. To do so, please edit the /components/com_jfbconnect/libraries/provider.php file. Around line 209, you should see:
$html .= $p->loginButton();
Update that to:
$html .= $p->loginButton( array('buttonType' => 'icon_button') );I hope that helps get you what you're looking for, for now. Should you need anything else or have other recommendations, just let me know.
Thanks,
Alex