The Moodle logs contain some great data on user activity. Awhile ago, I put a page detailing some statistics on WSD Online, our own Moodle server. You can find that page here: http://www.weber.k12.ut.us/index.php?page=moodle-statistics
Here’s a few MySQL queries you can use on your own Moodle server to get some statistics on the most popular activities, the most active users, and the most active courses. These have been tested with Moodle 1.9. I’m not a MySQL master, so they could probably use some further optimization, but overall they should work well enough:
Find the most popular activities:
SELECT COUNT(l.id) hits, module
FROM mdl_log l
WHERE module != 'login' AND module != 'course' AND module != 'role'
GROUP BY module
ORDER BY hits DESC

Find the most active users over the past 7 days (change the “604800” to the number of the appropriate number of seconds if you want to adjust this interval):
SELECT COUNT(l.id) hits, l.userid, u.username, u.firstname, u.lastname
FROM mdl_log l INNER JOIN mdl_user u ON l.userid = u.id
WHERE l.time > UNIX_TIMESTAMP(NOW()) - 604800
GROUP BY l.userid
ORDER BY hits DESC

Find the most active courses:
SELECT COUNT(l.id) hits, l.course courseId, c.fullname coursename
FROM mdl_log l INNER JOIN mdl_course c ON l.course = c.id
GROUP BY courseId
ORDER BY hits DESC

(You may need to change the second line to FROM mdl_log l INNER JOIN mdl_course c ON l.course = c.id AND c.id != ‘1’ to omit home page hits)