I got a bit bored and have heard occasionally people saying that the player base isn't growing anymore or that it's shrinking (and as shown it's staying about the same really). It's only for the forums since I'd have to dedicate and entire weekend to online arena stats and this is more of the Pie-covered-Snail's area, but I know some people love their little ol' charts. Maybe Snaily will do something with active players in the arenas (best way I can see it happening would still not include a large portion of players though), also there are over 1600 active players in LW alone last month according to the scores page (assuming there is a player for every #). On to Charts!
This is starting from Feb 08 when the boards got reset (and Feb had no stats really), based on a per month stat:

An average of ~228 new members per month per month.*

An average of ~34,047 per month.*

An average of ~15 posts per topic.*
*For anyone who forgets, average is all the Data from X divided by the amount of data series. Ex: 5,000 members divided by 32 months.The spikes in "Most online" correspond with updates, notably the March 2010 one is when WWI was released, and the October 2009 was Version 2.16 (still unsure what happened in 2.16 as the announcements don't say but I believe it was the P-47M as 2.17 fixed issues with that plane). Around June 2009 is when they implemented the new Terrain change with the beta testing.
Once everything steadied out after the Bulletin Board reset it's stayed about level since then with some drops and some spikes, and this is only the forums and I know plenty of people don't use them that play the game.