Check this out:
http://code.google.com/p/pidgin-facebookchat/
Here are the steps to follow:
http://code.google.com/p/pidgin-facebookchat/wiki/How_To_Install
After setting up, restart Pidgin and go to Manage Accounts and now there should be the Facebook account option
I got it working in Pidgin Portable 2.5. It works great.
http://code.google.com/p/pidgin-facebookchat/
Here are the steps to follow:
http://code.google.com/p/pidgin-facebookchat/wiki/How_To_Install
After setting up, restart Pidgin and go to Manage Accounts and now there should be the Facebook account option

I got it working in Pidgin Portable 2.5. It works great.

I think with Pidign 2.5 it have some Compalibility Probs 