having recently acquired a facebook, my current answer is probably an hour or two a week. My interaction with facebook is posting links of stuff I'm releasing (which, at the moment, has a target audience of people at my uni, when this project is more general, I'll post it on HN), answering friend requests, and answering messages posted to me.
Of those three, two of which I do after facebook sends me an email telling me either "hey, this guy requested to be your friend", or "hey, this guy sent you a message, here is the content: [...]".