I think it's pretty much been a steady decline. That's just what I think though. There's still plenty of people playing the game though. Thousands of players per server and many many servers.

I do think WoW affected the player population as I know people DID leave FFXI for WoW. But I think the biggest thing is just the fact that this is an old game. It's constantly being updated, but I can see way people would just want to call it quits once they'd been playing for multiple years and completed a lot of what they'd set out to do.