Taking advantage of the April Fool's Day all-posts-count, I've updated the list, checked a few things. I believe this is completely accurate (as of 1/4/14 at least).
5000+: Adventurer
4000+: Governor
3000+: Cartographer
2500+: Margrave
2000+: Mountebank
1500+: Torturer
1000+: Saboteur
750+: Jester
500+: Minion
450+: Witch
400+: Tactician
350+: Duke
300+: Explorer
250+: Apprentice
200+: Conspirator
175+: Golem
150+: Moneylender
125+: Young Witch
100+: Bishop
90+: Thief
80+: Spy
70+: Navigator
60+: Salvager
50+: Baron
45+: Coppersmith
40+: Scout
35+: Alchemist
30+: Ambassador
25+: Steward
20+: Chancellor
15+: Swindler
10+: Pearl Diver
5+: Herbalist
1+: Pawn
0+: Newbie
(Edit: Added Newbie)