All of those systems apart from IM follow the same route to success - a large number of independent systems with a common interchange format that makes it possible to create the illusion of integration.
Every mail server, every news server, every blog is a little island that runs on it's own. I would bet that behind the scenes even the big IM networks are actually a mountain of small, effectively independent, service providing systems that understand how to talk to each other - that's certainly how the XMPP/gTalk network functions.
This is only possible because all of those designs are fundamentally highly parallel - if your communication method has data dependencies (real or the result of bad design) that cannot be managed in that way then you get something that won't scale.
your opinion about sending messages being a very hard problem
I said scaling Twitter must be a hard problem, and that it will involve correcting their implementation rather than any one magic bullet. I said nothing about IM, email, blogs, etc.
You've decided that I'm talking about something that I am not.
Obviously you're entitled to your opinion about sending messages being a very hard problem, but I think it's been solved a few times now.