Include your signature before Jetpack's sharing buttons

inc-signature.php

<?php

/**
 * When hooked to either the_content or the_excerpt filters, 
 * this should append the signature before the Jetpack sharing buttons 
 * are added, so long as the priority is 18 or less!
 * (as Jetpack's sharing options go in at priority 19)
 * 
 * @param string $content The post content as passed to the function
 * @returns string The content with the author box appended to it.
 */
function blogaholic_post_signature( $content = '' ) {
	// Your signature function
}
add_filter( 'the_content', 'blogaholic_post_signature', 18 );