Just as an FYI, you can setup your own XMPP server in 20 min, and chat with people on facebook, gtalk, etc. www.prosody.im
Cue walled garden rant.
Wha you can do is use an existing Facebook (Jabber) account on a regular Jabber client such as Pidgin, Adium and such.
No problem chatting with @gmail.com JIDs from any Jabber server though (well, besides the fact that Google's Multi User Chat support is pretty shitty in my experience, and that the two-way Jabber subscription process it bit weird in Google land).
I absolutely second the Prosody advice though, wonderful server and very friendly community and devs!
S2S federation means server-to-server. That is, use your XMPP account on your own server (such as prosody, as proposed above) to communicate with facebook users (on facebook's servers).
But that would go against fb's walled garden approach, so it's unlikely to come.