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.