To be clear, bitlbee sits in between chat services (like AIM, ICQ, qq, various xmpp-based ones like gchat / facebook chat) and your irc client.
MSN/AIM/qq/XMPP/etc <-> bitlbee <-> your irc client
so you tell your irc client to connect (typically) to localhost where bitlbee is running, and bitlbee acts as a bridge between your irc client and various chat services all at the same time.
There's also irssi-xmpp, a plugin that does a slightly better job at exposing xmpp functionality to irssi, imo. Unfortunately it does not seem to be under active development. :(