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.
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.