Here is an answer I posted on Quora.
I am betting what we know of email will become more of a notification service plus a way to communicate with people outside our regular networks.
Facebook and Google+ are our networks of our friends and family. Twitter is our network of interests and associations. LinkedIn is our network of professional contacts. And project workflow tools (such as Asana, Trello, LiquidPlanner, Basecamp, MavenLink, etc.) are networks of people in our projects. Instant Messages work for our closest friends/family/coworkers with whom we have the permission to interrupt them.
We will want more of our communications to be "in context" with our particular network. That will lead to appropriate sharing and limit the number of documents emailed once and hidden on some person's hard drive.
Gmail already facilitates this separation of types of emails with their tabs -- to keep primary correspondence separate from network notifications and from email promotions/SPAM.
This blog about AngelList says it well: No email at AngelList. Note the instances of email:
First, when we need to communicate with people outside the company.
Second, when we need to have a conversation with an ad hoc group of people inside the company. HipChat is not great for ad hoc groups that only need to discuss a single task like, “how should we negotiate this deal.”
Third is laziness and stupidity (guilty).
The challenge will be to undo the reliance on business email among +/- 30-40 year olds who started in business with email so prevalent but who are also reluctant to adopt new tools. They are today's and tomorrow's business leaders, and will set the tone for many years.