Since I assume you're referring to posts more so than just site traffic, here are some stats.
The month leading up to the upgrade -
Code:
The Lounge 8928
Archive - The Lounge 8330
European & African Drug Discussion 8270
Psychedelic Drugs 5265
Other Drugs 4667
The month following the upgrade -
Code:
European & African Drug Discussion 6746
The Lounge 5515
Psychedelic Drugs 5330
Other Drugs 4481
Basic Drug Discussion 3310
Average month in the 6 months leading up to the upgrade-
Code:
Archive - The Lounge 10919
The Lounge 9400
European & African Drug Discussion 8518
Psychedelic Drugs 6583
Basic Drug Discussion 6547
EADD's posts are climbing though...
The previous month up until today (about 3 weeks of this overlapping the month follow the upgrade stats) -
Code:
European & African Drug Discussion 7456
The Lounge 6555
Psychedelic Drugs 5323
Other Drugs 4463
Basic Drug Discussion 3862
It seems there was somewhat of a hit in posts in some of the busiest forums but as dok pointed out, overall site traffic is up. This is ONLY the top 5 most posted in forums however and is not necessarily representative of other forums. The upgrade probably did play a role but it's hard to say how large of one and how much other factors played in.