Web-project "Session End"

Hello, i work with web-game for some social networks, i wrote custom analytic server to have feedback and to know how fine is my game. My system have many analytic-resources and all works good, except one feature. I always save users session progress (what did he do, how long was session lifetime etc.) and usual, in not-web applications i use "OnApplicationQuit"/"OnApplicationPause" methods to know when should i close current session (because player quit the game).

But now, with web-app i have problems with it, because players don't use any triggers to "close" web-game, they just close their browsers or insets with application, and i have no idea how can i catch "application close" event to make my analytic system work fine.

So, it would be great if you have some ideas and can help me with it. P.S. Thanks a lot and sorry for my bad English =\