Those buttons are placed there by JomSocial. If you want to change the position, you will have to create a template override of their registration page.
The file you want is the /components/com_community/templates/jomsocial/layouts/register/base.php (or substitute "jomsocial" folder for whatever template you're using). Around line 107 is the following block of code that you can move up:
<?php if ( $fbHtml ) { ?>
<div class="joms-form__group">
<span></span>
<?php echo $fbHtml;?>
</div>
<?php } ?>
After you make the code change, I suggest you make a new JomSocial template with the changes so that they don't get lost when you upgrade JomSocial in the future.
-Melissa
Facebook login on JomSocial registration screen
