Facebook Development API allows desktop apps to be created and has a code library for them. I’m surprised no one has created an Instant Messenging application (like AIM, Yahoo! Messenger, MSN Messenger, etc) that uses Facebook users — obviously your friends list, but also live chat rooms too.
There is supposedly a Facebook web app for IM, but all the user comments on it make it sound as though it doesn’t work.
You could also likely write a mobile app (iPhone, new wifi iPod, etc) that is a Facebook IM client.