I am working on a multiplayer chess game as a Facebook app.
If one player leaves the game by closing the browser the other player should get a notification. So if one player closes the Browser, a unlink function should be called to unlink the player. This works fine with onunload outside Facebook.
The problem is, that the Facebook apps are loaded in iframes and the onunload event doesn't work there.
So I need a way to call a function inside an iframe when a user is closing the browser.
This is probably not the answer you are looking for but "logging off" on unload will never work reliably. For an extreme example, consider the case where the browser crashes or is killed via the Windows task manager. So you better implement an additional mechanism to detect whether a user left. Typically this is done by sending a request to the server periodically. If this request isn't received for a significant time (meaning something that cannot be caused by a slow connection or other hiccups) you unlink the user.
That said, I tested Firefox 4 and MSIE 8 and both correctly fire unload event on the frame if the tab or the browser is closed. Chrome 12 doesn't do that, that's probably the browser you have been testing with. I consider it a Chrome bug.