After trying to hit Safari's Back button but instead closing all the windows, a crazy theory occured to me: The rise in instant messaging is fueled by as much by fragility of webmail (and web applications in general) as it is by the other improvements over email (specifically, presence.)
Specifically, the younger crowd that is most drawn to IM is exactly the one least likely to have Outlook or a regular mail server and thus using Hotmail or Yahoo! mail. Since web applications aren't first-class desktop citizens they therefore are missing several interfaces that normal applications have: you cannot Alt-Tab to them, you cannot launch them separately, they don't "install" and stick around on your desktop, and notably, they cannot control what happens when you close them. Indeed, you'll notice that instant messaging applications from every provider forgo even the usual window decorations and are very hard to quit indeed. Another example which attempts to reverse this trend is Pyro, an appification of the group-chat webapp Campfire. And Firefox will initially balk at being closed when it has more than one tab open.
I think before long we'll see a generic way to dress up web applications much like normal desktop apps but still retain much of their own power; I just hope it's sooner rather than later.