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I've used Trillian Astra and Miranda for multiclient support, but I don't know if they support Facebook now or not. Now that I've moved to OSX, I use Adium - genius.
Adium is a great Pidgin alternative for Mac OS X.
They both use libpurple (coded for Pidgin)
Pidgin is available for *nix, Windows and OS X. So you should probably look into it.
Trillian Astra has an interesting history, although it's not freeware.
Yea. i was going to recommend both pidgin and astra. trillian if your into resource heavy stuff, pidgin is the low resource alternative. I like pidgin myself.
im liking pidgin. used it on linux and loved it. it crashed several times when I tried to change smileys, and crashes when I set my status to offline, but those are minor issues.