I don't care what site you join personally. I don't think posts should be deleted that contain numerous links though Jave. That's kinda wack.
To answer the OP's question - Nasioc would probably be the "most active" WRX forum on the net even though it handles all Subarus. They have hundreds of thousands of members and tons and tons of traffic every day. Newbs usually get ripped apart though. IWSTI.com has similar figures but they are devoted to the STi only.
This site we're now conversing on currently has about 172k posts (not sure if that counts off topic stuff or what though) since it was first created in I want to say 2001 just before the 2002 WRX hit US shores (but it might have been earlier). Through very good (or very questionable) search engine optimization it is almost always on the first page of google and yahoo searches pertaining to WRX, Subaru, etc.
WRXModders has about 140k posts since it was created in the Summer of 2006 (so it's a little over a year old) and it sees countless new members all the time. They don't come up as frequently as WRXTuners does in searches, but that community has been spreading better through word of mouth.
There are other sites too but I'm usually pretty disappointed in them. Some people really like i-club which isn't bad. Really I'd just get accounts on all of them and check out the conversation. Every site has a different environment.
Patrick - 2007 STi Limited