Message volume shouldn't be a problem for services like Twitter. RabbitMQ with an XMPP gateway might be part of the the solution. The combination means you can inject very high volumes of messages into IM or other XMPP enabled applications with far fewer scale issues.
For the geeks:
The mod_rabbitmq module implements an ejabberd extension module which gateways AMQP (as implemented by RabbitMQ) to XMPP.
Via James Governor Monkchips on Twitter.