If only the channel allocation scaled..
PSYC would be using IP Multicast if the channel allocation had a chance to scale.
<lynX> Around 1996 I discussed a lot with the MBONE people on how we could make it, that IP Multicast scales well enough so that every little chat channel can travel over it. PIM/SM was supposed to address that, but even that requires a worldwide router broadcast to allocate a channel, which is completely inacceptable as a large scale procedure. Not sure if this is still the state of things, I haven't followed up on it in a while.
But I am pretty sure you cannot allocate a Multicast IP for every person to send her presence, which is just a clue of what would be necessary.
So we're digging up our own multicast strategies. Sigh.
In the following slides XCAST makes the right criticism of the problem:
Most technology people do not even know what Wikipedia:IP Multicast was originally intended for. It has become best known as an alternative way to do LAN broadcasts. Ouch!
See also Scalability.