They only support client to server XMPP, not server to server XMPP.
Facebook Chat should be compatible with every XMPP client, but is not a full XMPP server.
So you can use any XMPP client for the Facebook Chat, but you can only chat with other Facebook users.
Which is a pity. Welcome to the 90s.
In my recent experience contacting friends who use GMX's xmpp service works pretty unreliable. Only friends with a gmx.net|gmx.com|gmx.de|gmx.at|gmx.ch address can be reached OK. They are routed via xmpp-gmx.gmx.net[18.104.22.168]. Friends with a gmx.biz|gmx.info|gmx.name|gmx.org|gmx.eu|gmx.tm|gmx.co.uk|gmx.ie|gmx.fr|gmx.lu|gmx.li|gmx.it|gmx.se|gmx.pt|gmx.es|...
A Facebook account comes already with a Jabber ID, firstname.lastname@example.org and the password is the same as your account.
You can find it in the profile; it's called "username" or something like that.
However, a few months ago when you try to use and external XMPP client, Facebook will alert you to allow such type of connection. And now you can't use any ...
go to your Contacts: https://contacts.google.com/
choose/find a name
click on the 3-dots (on the right side of selection)
then go to Gmail: https://mail.google.com/mail/u/0/
click on Hangouts' icon
click on 3-dots
The best thing to do then is to hide them.
If you are using the mobile app, then touch and hold on that contact (if visible) and touch on hide contacts. Then if you want to view it, go to settings. There you will find all the hidden contacts.
Please verify that your entry in $HOME/.centerim/config matches this example.