1. 17.81 The Whelk
2. 13.03 Artw
3. 10.12 DU
4. 9.75 hippybear
5. 9.70 Burhanistan
6. 9.60 delmoi
7. 9.50 Blazecock Pileon
8. 9.46 EmpressCallipygos
9. 9.40 empath
10. 9.03 ericb
11. 8.39 Ad hominem
12. 7.73 cortex
13. 7.46 the man of twists and turns
14. 7.25 klangklangston
15. 7.10 Sys Rq
16. 6.96 filthy light thief
17. 6.79 homunculus
18. 6.70 flapjax at midnite
19. 6.67 Brandon Blatcher
20. 6.64 jessamyn
21. 6.57 griphus
22. 6.48 amberglow
23. 6.36 rtha
24. 6.23 Lovecraft In Brooklyn
25. 6.19 KokuRyu

1. 1224 homunculus
2. 1188 mathowie
3. 870 Artw
4. 648 stbalbach
5. 605 zarq
6. 602 jonson
7. 599 filthy light thief
8. 563 Postroad
9. 550 fearfulsymmetry
10. 534 y2karl
11. 529 netbros
12. 526 flapjax at midnite
13. 509 Kattullus
14. 486 madamjujujive
15. 460 MiguelCardoso
16. 456 crunchland
17. 433 The Whelk
18. 381 Steven Den Beste
19. 367 kliuless
20. 351 Blazecock Pileon
21. 351 Brandon Blatcher
22. 349 gman
23. 346 empath
24. 346 owillis
25. 341 quin

1. 33221 delmoi
2. 32002 Artw
3. 26165 homunculus
4. 23519 ericb
5. 20667 The Whelk
6. 20516 DU
7. 19471 jonmc
8. 19236 amberglow
9. 18143 empath
10. 17020 five fresh fish
11. 16598 Blazecock Pileon
12. 13692 dhartung
13. 13156 hippybear
14. 13090 Burhanistan
15. 12596 flapjax at midnite
16. 11991 Astro Zombie
17. 11741 klangklangston
18. 11675 mrgrimm
19. 11343 koeselitz
20. 11225 Smedleyman
21. 10592 languagehat
22. 9812 shakespeherian
23. 9686 pyramid termite
24. 9407 quin
25. 9351 Pope Guilty

Excellent question. It is simply the sum of all the times a user has posted on Metafilter (comments or threads on the blue, gray and green) 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.