How to shoe last active users

I did this to show the latest users who have ha registered on my website:

<?php
$usernames = $wpdb->get_results("SELECT user_nicename, user_url FROM $wpdb->users ORDER BY ID DESC LIMIT 18");
foreach ($usernames as $username) {
	echo get_avatar($username,60);
    //echo '<li><a href="'.$username->user_url.'">'.$username->user_nicename."</a></li>";
}
?>

Now what I want is to show the latest users who were active.What would be the right approach to go with?