instead of
// Add a header hook so we can link to a CSS file
add_action('wp_head', 'most_shared_posts_head');
// Our header function which'll hook in our CSS file
function most_shared_posts_head() {
echo '<link rel="stylesheet" type="text/css" href="' . plugins_url('most-shared-posts.css', __FILE__). '">';
}shouldn't you use
// Add a header hook so we can link to a CSS file
add_action('wp_enqueue_scripts', 'most_shared_posts_style');
// Our header function which'll hook in our CSS file
function most_shared_posts_style() {
wp_enqueue_style( 'most-shared-posts', plugins_url('most-shared-posts.css', __FILE__) );
}