IMs hit the sweet spot between fully real-time (like the phone) and fully async (like paper mail).

The UI is built for efficient real-time communication, but the underlying data model (the log, notifications, statuses, etc) allows for very easy async communication.

I think someone could make a great product by just adding IM-like UI features on top of an existing email backend. Email is pretty fast these days to work as mostly-instant communication.