So what would it take for a product like this to replace email?

With email, it doesn’t matter what email provider that user is using or what type of email server the provider is running, it is easy to email other people. Unlike social networking where I may need several social networking accounts to contact everyone I know.

Email is pretty light weight, I can view and send my email using webmail clients, desktop clients, mobile devices natively, could even use a telnet client.

For a system like this to take off it woud need to be cost efficient to implement. It would need to be able to work with existing email systems and existing authentication methods. It would probably need to be administered in similar ways to existing communication platforms. You need to be able to have data exchange between various 3rd party systems. You need to be able to use the system out side the browser, on software clients, mobile devices etc. You need to be able to offline.

Once you add the above functionality some of the shortfalls of email start to come back.