JFBConnect recently added a way to add a (translatable) text string to the login buttons, but JomSocial isn't currently using it.
In the meantime, I'd recommend editing their frontpage template file, at /templates/default/frontpage.guests.php. At the bottom, you'll see:
Update that with:
<?php echo JText::_('COM_JFBCONNECT_LOGIN_WITH');
echo $fbHtml;?>The COM_JFBCONNECT_LOGIN_WITH string can be found in the /language/en_gb/en-GB.com_jfbconnect.ini file. By default, it's simply "Login with ", which would then be followed by the social buttons.
I hope that helps, but if you have any other questions, just let me know.
Thanks,
Alex