Updated Oct 25, 2016
Excellent question. It is simply the sum of all the times
a user has posted on Metafilter (comments or posts in any section of MeFi)
divided by the number of days they have been a member.
Tamim's user profile discusses the idea and while reading this discussion I saw moz's reference to tamim's user page. After reading that and seeing tamim's suggestion, I wrote a quick script to do it. If you see any problems please let me know.