Add extra social media sites to Social Media Widget

functions.php

<?php // Do not include in an existing file.

add_filter( 'jetpack_social_media_icons_widget_array', 'bk_social_media');

function bk_social_media( $html ){
$html[] = '<a title="View my Tumblr" href="https://example.tumblr.com" class="genericon genericon-tumblr" target="_blank"><span class="screen-reader-text">View my Tumblr</span></a>';
$html[] = '<a title="View my Flickr" href="https://flickr.com" class="genericon genericon-flickr" target="_blank"><span class="screen-reader-text">View my Flickr</span></a>';
return $html;
}

// The `genericon-tumblr` class name should be replaced with the proper one from http://genericons.com/ for the icon desired.