As far as I know, AIM still has 10x the users of MSN Messenger.
The story I hear is that MS started bundling MSN Messenger in WindowsME, set to launch at boot time by default, so anyone with a HotMail or MSN account who installs WindowsME automatically becomes an MSN Messenger user. This did wonders for their user base, just as it did for Internet Explorer's user base in the late '90s.
But I have only heard this by word of mouth; I'd also appreciate seeing a reference to it from some reputable source.
�Jens
