There's no technical reason why a decentralized system can't work. The main road blocks would be finding a compelling reason for people to switch away to one in the first place.
Email is decentralized. It seems to have worked pretty darn well.
Email is just as much a strawman. Compare how simple email is to all of the features Facebook provides (and don't forget your ability to click on your cows in Farmville).
You say there are no technical reasons why it can't work. That is very akin to the "You can do anything, it's just a simple matter of coding." The reality is that some problems become intractable due to their complexity. Distributed social network may not be intractable (Diaspora will probably figure that out), but it's a hell of a lot harder than email.
Just to clarify, when I said 'technical reason' I meant from the perspective of a user being able to setup/configure their social network settings. Not from the perspective of the internal implementation of the server components
Most people do. Not all.
The point being that you could choose to roll your own if you wanted to.
I bet many people on HN have their own domain and their own mail server. Hell, for a few dollars a month you can get a web hosting company to set it up for you.
For those who care about those things, they can do that and still be able to communicate with people who use gmail or yahoo mail.
