Update: MG Siegler (TechCrunch) has a post along similar lines, asking for a "Gmail Lite".
What Facebook realized was that the world didn’t need another email platform but a better, more simple way to stay connected with the people who count the most in our lives – our friends. So Facebook boils messages down to just two things: friends and their messages. There is a simple idea behind this approach to communications:
Friends define priority.
I also think it's critical to consider how the next generation is growing up in a 24-7 connected world and that their norms and expectations just don't work with email's asynchronous-by-design behavior.
So, if we changed the form factor of email from this:
...to look like this?
...we'd be closer to how people use chat and IM and we'd likely change the style of conversation that occurs.
With sites like Sentenc.es, evidence suggests that many of us struggle with email overload. As a result, we just don't have enough time in the day to respond to longer messages with long messages. Restricting message content with Twitter-like brevity is the only way to multiplex our communications without having to reduce the number of people with whom we interact on a regular basis.
I think this is Facebook's realization, and the key insight that drove the design of Facebook Messages.
This new modality — of "continuous partial conversations" — means that we can keep talking even if we change devices or are in a computing mode that's not synchronized with our friends.