Add social share links in WordPress RSS feed. Source: http://www.wpbeginner.com/wp-tutorials/how-to-add-social-buttons-to-wordpress-rss-feed/

add-social-share-links-in-rss-feed.php

<?php

function prefix_add_social_share_links_in_rss_feed($content) {
    if (is_feed()) {
        $permalink_encoded = urlencode(get_permalink());
        $post_title = get_the_title();
        
        $content.= '<br><br>Share: 
        <a href="http://www.facebook.com/sharer/sharer.php?u=' . $permalink_encoded . '" title="Share on Facebook" target="_blank" rel="nofollow">Facebook</a> • 
        <a href="https://twitter.com/intent/tweet?hashtags=customhashtag&text=' . $post_title . '&amp;url=' . $permalink_encoded . '" title="Share on Twitter" target="_blank" rel="nofollow">Twitter</a> • 
        <a href="https://plus.google.com/share?url=' . $permalink_encoded . '" title="Share on Google+" target="_blank" rel="nofollow">Google+</a> • 
        <a href="http://www.linkedin.com/shareArticle?mini=true&url=' . $permalink_encoded . '" title="Share on LinkedIn" target="_blank" rel="nofollow">LinkedIn</a> • 
        <a href="mailto:?Subject=' . $post_title . '&amp;Body=Your%20custom%20text%20goes%20here:%20' . $permalink . '">Email</a> • 
        <a href="https://getpocket.com/save?url=' . $permalink_encoded . '" title="Save to Pocket" target="_blank" rel="nofollow">Pocket</a>';
        
    }
    return $content;
}
add_filter('the_excerpt_rss', 'prefix_add_social_share_links_in_rss_feed');
add_filter('the_content', 'prefix_add_social_share_links_in_rss_feed');