All I know is that 600,000 people left after Promise. They've never had such a significant drop in the other season, if I'm not mistaken.
I was curious about this too, but in fact, they have, several times...
S4:4.Devoted........final number 6.2 million viewers. (airdate: 10/13)
5.Run............final number 5.4 million viewers. (airdate: 10/20)
6.Transference......final number 5.7million viewers. (airdate: 10/27)
7.Jinx...............final number 5.0 million viewers. (airdate: 11/3)
10. Scare ............final number 4.9 million viewers. (airdate: 12/1)
HIATUS
11.Unsafe......... final number 4.2 million viewers. (airdate: 1/26)
15. Sacred ..........final number 5.3 million viewers.(airdate: 2/23)
16. Lucy.............. final number 4.5 million viewers.(airdate: 3/2)
HIATUS
17. Onyx........... final number 3.9 million viewers.(airdate: 4/13)
Though it should be noted that the numbers kept bouncing back, and the season ended with a 5.5million viewers. So, the numbers fluctuated, but there was no steady decline
S5:4. Aqua.......final number 6.4 million viewers.(airdate: 10/20)
5. Thirst.....Final Number 5.8 million viewers (airdate: 10/27)
(though I think this one has to factor in the huge bounce "Aqua" got)
8. Solitude......Final Number 6.0 Million Viewers (airdate: 11/17)
9. Lexmas........Final Number 5.4 Million Vewiers (airdate: 12/??)
12. Reckoning.....Final Number 6.3 million Viewers.(airdate: 1/26)
13. Vengence.....Final Number 5.4 million Viewers.(airdate: 2/2)
(Again, I think you have to factor in the huge bounce for the 100th episode hype)
15. Cyborg....Final Number 6.2 million Viewers.(airdate: 2/16)
HIATUS
16. Hypnotic...Final Number 4.8 million viewers.(airdate: 3/30)
17.Void...Final Number 4.2 million viewers.(airdate: 4/6)
18. Fragile......Final Number 3.9 million viewers (airdate: 4/13)
S5 had a small slide towards the end, but bounced back after that and ended the season with a 4.8million, then a 4.9million. But it clearly never got back up to early S5 strength, which S4 did.
S6:16. Final numbers for Promise...................4.7 million viewers.(airdate: 3/15)
17. Final numbers for Combat..................4.1 million viewers.(airdate: 3/22)
BREAK
18. Final numbers for Progeny......................4.0 million viewers (airdate: 4/19)
19. Final numbers for Nemesis......................3.9 million viewers. (airdate: 4/26)
20. Final numbers for Noir......................3.6 million viewers.(airdate: 5/3)
21. Final numbers for Prototype......................3.4 million viewers.(airdate: 5/10)
(credit for ratings info,
dec5 via Sweet )
What seems to be different in the S6 slide was that it was steady, maintained slide and the show never really recovered from it. There was no bounce back, which happened in both S4 & S5. Though "Phantom" did get back in the 4million range, thats still a pretty sad number for a finale.
So, what does all that mean? I have no clue. But the show has had huge drops before. Though I'm not sure it ever had a slide in the 2nd half of the season quite like S6 had.
JMHO