I think Facebook supports the open standard XMPP/Jabber instant messaging protocol, so it should be possible. There's a jabber client for MorphOS called Jabberwocky, you can find the latest version here : http://sourceforge.net/projects/ajabberwocky/files/Jabberwocky/1.9/

here are the settings for Facebook :

domain/server : chat.facebook.com
user : {your facebook user}
password : {your facebook password}
port : 5222
you may have to disable SSL/TLS

i haven't tried myself, and i'm too lazy to even have a look at it, maybe it's not even supported but worth trying.
Otherwise Pidgin on linux can connect to Facebook chat, so it should be too hard to look at the source code.