Should use wp_enqueue_style

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__) );
}

http://ww.wp.xz.cn/extend/plugins/most-shared-posts/