First I'll conceptualize the process, then we'll go straight into setting it up on an Ubuntu 9.10 system.
Basically it's IRC client --> tircd IRC twitter gateway --> twitter --> facebook's Selective Twitter Status application --> facebook
I decided to use the Selective Twitter Status application because I use twitter too and don't want all my tweets to totally spam my friends like a bad EFnet bot. (you IRC veterans know what I'm talking about)
Also, take note of the port change to 6668 if you use bitlbee for IM.
sudo aptitude install tircd
Install Selective Twitter Status
http://apps.facebook.com/selectivetwitter/ & enter your twitter username
Configure tircd (optional)
gksudo gedit /etc/tircd.cfg
chage line:9 from port 6667 to port 6668
terminal: tircd &
run dialog: tircd
Update facebook Status
open preferred IRC client
/SERVER localhost 6667 [twitter_password] [twitter_username]
/SERVER localhost 6668 [twitter_password] [twitter_username]