1. 9.09   Artw
2. 8.51   The Whelk
3. 6.88   EmpressCallipygos
4. 6.06   hippybear
5. 5.98   cortex
6. 5.09   jessamyn
7. 5.06   delmoi
8. 4.84   praemunire
9. 4.77   homunculus
10. 4.59   empath
11. 4.58   filthy light thief
12. 4.48   Anonymous
13. 4.31   flabdablet
14. 4.15   Brandon Blatcher
15. 4.1   klangklangston
16. 3.96   ericb
17. 3.94   Miko
18. 3.86   Justinian
19. 3.84   DU
20. 3.81   rtha
21. 3.59   Sys Rq
22. 3.57   griphus
23. 3.55   Sara C.
24. 3.5   zarq
25. 3.49   languagehat

1. 2167   Fizz
2. 2055   filthy light thief
3. 1688   the man of twists and turns
4. 1515   homunculus
5. 1505   hippybear
6. 1430   Artw
7. 1329   The Whelk
8. 1298   mathowie
9. 1092   chavenet
10. 1061   MartinWisse
11. 1036   Johnny Wallflower
12. 1020   fearfulsymmetry
13. 1001   Etrigan
14. 976   zarq
15. 951   kliuless
16. 894   Kattullus
17. 742   flapjax at midnite
18. 737   roomthreeseventeen
19. 730   stbalbach
20. 714   y2karl
21. 650   adamvasco
22. 650   Brandon Blatcher
23. 643   Postroad
24. 627   jonson
25. 598   Chrysostom

1. 55500   Artw
2. 36270   The Whelk
3. 36049   homunculus
4. 33791   delmoi
5. 29068   hippybear
6. 23518   ericb
7. 23028   EmpressCallipygos
8. 22246   Justinian
9. 21507   jonmc
10. 21397   empath
11. 21244   DU
12. 19236   amberglow
13. 18608   five fresh fish
14. 18386   filthy light thief
15. 17432   JHarris
16. 16967   zachlipton
17. 16712   klangklangston
18. 15698   Sys Rq
19. 15631   dhartung
20. 15431   Pope Guilty
21. 15264   srboisvert
22. 15177   Thorzdad
23. 15077   octothorpe
24. 14846   kirkaracha
25. 14690   GuyZero

1. 528   mathowie
2. 252   cortex
3. 197   Eyebrows McGee
4. 173   MiguelCardoso
5. 125   jessamyn
6. 124   taz
7. 110   Fizz
8. 109   WCityMike
9. 105   pb
10. 105   signal
11. 101   stavrosthewonderchicken
12. 79   scarabic
13. 79   stupidsexyFlanders
14. 78   dobbs
15. 77   feelinglistless
16. 74   empath
17. 74   wendell
18. 71   ParisParamus
19. 71   grumblebee
20. 68   timeistight
21. 68   sgt.serenity
22. 66   desjardins
23. 64   drezdn
24. 63   crunchland
25. 61   ThePinkSuperhero

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.