What's wrong with email?
I think email is still relevant only for a document trail for business. If its about leaving a document trail, maybe there should be something new and more suited for it - for approvals, discussions, etc.
Its not like people are looking for a solution for a "problem", but then someone is going to create that something - which will make email less relevant even for the business use.
Messaging - email, fb, linkedin, etc
Broadcasting - twitter, fb wall, rss, etc
Instant Messaging - skype, irc, etc
Calling - skype, phone, etc
In Person
That people are doing more messaging through fb, linkedin etc doesn't change that what they are doing is essentially email.
FB - how much ever they make it look like email and function in a compatible manner, I don't think its used the same way email is(talking user behavior). It is just an asynchronous text messaging system. It is closer to chat/IM than to email in the way it is used. FB's bid to make it email-like seems to be an effort to move more of your messaging to FB or have the "unified inbox".
Is it really tough to imagine that 10 years from now we won't use "email" for more than 5% of our communication? And that subsequent generations will probably not use it at all?