When I mentioned IM, I was referring to the fact that service to service communication is closed. With the exception of the XMPP network and some one-off deals between some of the big services, services to not communicate with each other. There is not a single federated instant messaging network.
There is a single federated network for email. I can send a message from my account on Hotmail to Gmail without having an account on Gmail. Imagine what the world would be like if we couldn't do that. We would all have accounts on several services, each with their own inbox. It would suck.
It would probably be little bit like needing an account on Facebook, Linkedin, Xing, Twitter, Myspace, Flickr, Google and a dozen other properties that barely - if at all - interact with one another.
It would suck.
Exactly.
Not for most users. They simply reuse the same login credentials at all the sites they use. Other sites make apps that allow data to flow into Facebook easily.
Soon all Facebook users will be able to maintain their contact information over a variety of methods (e-mail, sms, etc.).
In the end, we already have a federated network that solves the problem for most people: Facebook.
There are now a plethora of instant messenger clients that allow for a single sign on to various instant messaging networks. I think we might see the same in the future for social networks. Facebook and Twitter will be relegated to the role that the various instant message protocols play in a multi-network client.