Shortcode in pages or tempate files for custom post type

Lucas Wynne linked some good plugins for such functionalities (I have used Addthis multiple times), however if you’re looking for a plugin-free solution, i’d more likely filter the content in my functions.php file instead of using a shortcode :

some pseudo-code

 function add_content( $content ){
     $content .= "<a href=\"facebook\">FACEBOOK</a>";
     return $content;
}

For more info on the content filter : http://codex.wordpress.org/Plugin_API/Filter_Reference/the_content