Nope, not on Facebook you can't. Unless I'm sourly mistaken, the Facebook Jabber server does not support s2s federation (by choice, obviously) and won't let users outside the Facebook Jabber server chat to those on it.

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!