A few months ago there were under 400,000 users. Now there are over 600,000. Where did they all come from? Plus when you have 10 armor points you are ranked like 200,000. Do people really make accounts that they never use?
A lot of people make ghost accounts for reasons unknown. Usually a high increase of users usually mean that there's a game that you can register for to get your name on the high score list. Around 2/3 of the AG populations don't even know that these forums exist. Everyday you can expect at least 25 people register because of the games. But every now and then there will be a user that will become active in the forums and will not spam. Which is probably every month...
Some people create an account to view feedback, to rate or leave feedback on a couple of games, etc. then leave.. I don't see why people would create accounts for no reason..
A lot more users than normal usually join when a highly popular game, such as Crush the Castle 2, is released. The fact that it was also summer most likely added even more to new membership. Exit Path, which allows for multiplayer games, was also released over the summer, but I'm not sure if it fits within the time frame.
You can also consider the amount of advertisement Armor Games uses from other sites in order to bring more users in. There is no doubt that Armor Games makes some of the most top-notch flash games out there. When they see that sword and shield, they know they are playing a good game. It's just one step further to click the link to become part of it.
CtC2 and Exit Path did make a significant increase of users over th summer and it was quite annoying that it did. Most accounts will probably never be used again until another highly anticipated game comes out like Colony 2 or Sonny 3.