It's only asynchronous is you have willpower, which is finite and easily exhausted, and if there's no expectation of realtime response on the other end, which there nearly always is these days. Whoever comes out with a viable system that is explicitly async (like mailman, or a good secretary) will really benefit humanity.
It's like normal Facebook, which I don't think even the biggest of addicts treat as a synchronous system. The entire structure of the application is for you to post and consume updates at different times.
You're going to have a hard time convincing me that Facebook is synchronous when the entire power of News Feed (and what makes Facebook so much more powerful than Facebook) is its ability to keep you updated without requiring constant real-time attention.
It does not require constant attention, but in fact it is addictng for users and people do sit and refresh their feeds for extended periods of time, watch the live activity stream on the side, or engage in chat, etc. Facebook wants you to spend a lot of time on Facebook and optimizes for this. They do not expect people to quickly check the site and leave and I would suggest most people don't do that. Or they they do, but they do it so repeatedly (continuous partial attention) it's even worse.
Edit: I would just add that in an environment where every site action has some kind of real time notification tied to it, the async nature/expectation is greatly diminished. Especially in a work context. And so now you have to add 'good job!' and water cooler type notifications to that cognitive overhead too.
Check out TMail21 ("Power Threads for Teams". It is explicitly designed as an asynchronous communication platform. Furthermore, it is being integrated into all of the chat services like Slack, Hipchat etc. to avoid balkanization between chat and asynchronous communication.
