1. 10.96 DU
2. 9.77 ericb
3. 9.73 delmoi
4. 8.89 Marisa Stole the Precious Thing
5. 8.65 amberglow
6. 8.45 klangklangston
7. 8.44 Joe Beese
8. 8.18 Blazecock Pileon
9. 8.10 The Whelk
10. 8.06 Artw
11. 7.52 cortex
12. 7.41 flapjax at midnite
13. 7.31 Astro Zombie
14. 7.17 Burhanistan
15. 7.07 languagehat
16. 7.02 jonmc
17. 6.54 five fresh fish
18. 6.27 Ambrosia Voyeur
19. 6.02 Mister_A
20. 6.00 caddis
21. 5.72 Smedleyman
22. 5.68 UbuRoivas
23. 5.61 desjardins
24. 5.56 homunculus
25. 5.45 bugbread

1. 1156 mathowie
2. 1075 homunculus
3. 602 jonson
4. 561 Postroad
5. 500 jaden
6. 495 y2karl
7. 460 MiguelCardoso
8. 417 madamjujujive
9. 394 crunchland
10. 381 Steven Den Beste
11. 346 owillis
12. 334 flapjax at midnite
13. 321 amberglow
14. 320 plep
15. 306 skallas
16. 305 kirkaracha
17. 300 hama7
18. 290 wendell
19. 286 srboisvert
20. 284 XQUZYPHYR
21. 264 matteo
22. 261 semmi
23. 257 Kattullus
24. 254 feelinglistless
25. 227 mr_crash_davis

1. 24608 delmoi
2. 19236 amberglow
3. 16212 jonmc
4. 15936 homunculus
5. 12202 ericb
6. 11643 five fresh fish
7. 10250 dhartung
8. 9778 languagehat
9. 9014 Artw
10. 8981 quonsar
11. 8660 Smedleyman
12. 8353 flapjax at midnite
13. 7768 y2karl
14. 7485 matteo
15. 7462 DU
16. 7194 ZachsMind
17. 7041 caddis
18. 6964 ParisParamus
19. 6838 stavrosthewonderchicken
20. 6661 mr_crash_davis
21. 6566 pyramid termite
22. 6385 troutfishing
23. 6056 Astro Zombie
24. 5959 cortex
25. 5953 kirkaracha

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.