I liked this mod except for the # count that was wrong.... I just wanted the people in the past 24 hours.
I edited his model and modified these two lines near the end.
$mydate = strtotime("-1 day");
$WHVArray = $db->fetchAll("SELECT user_id FROM xf_user as user WHERE last_activity >= '$mydate' ORDER BY last_activity DESC");
$count = $db->fetchOne("SELECT COUNT(*) FROM xf_user as user WHERE last_activity >= '$mydate'");
This may or may not work for you -but I wanted a little more accurate. I am not excluding banned, or Not Visible users.