In reality no one uses Jabber except me and like 2 other guys, and I'm just turned it on now for the first time, and I'm not really sure what it will actually do.
Jabber is the protocol used by Google's GTalk.
It's also supported by Miranda, my favourite IM client. Miranda's great 'cause you can run ALL your IMs in one window: MSN, ICQ, Gtalk, etc etc. It's also immune from that nasty MSN virus that's been going around.
In any case, it's active now, and I'll be using it myself 'cause it's even faster than the already-awesome RSS feeds which I'm sure no one but me uses.
Note! You can enable Jabber notification in your user profile.