Facebook has a jabber server, with this tool you can now chat with your Facebook friends via a jabber client (as is done now with messenger)
Jabber is an open instant messaging protocol and has already been Adopted by google talk
there are many jabber client and for almost all operating systems
windows / linux / mac is that Pidgin is multiplatform, multiprotocol that allows you to use multiple protocols together, for example, msn, yahoo, google talk, etc.
for more information and to configure your client go to the following link!
http://www.facebook.com/sitetour/chat.php
