Firstly, unfriend your boss!
I do have a mate who has multiple friend categories on FB (so her relationship angst doesn't go to her mum, her work grumbles aren't seen by her boss), but if you CBA with that, just get rid & block. Allow them to think it's because you've stopped using your account.
Secondly, everyone knows that if you are well enough to update FB, you are well enough to be at work down t'pit/pole dancing/running the country. Or something.
A workmate of mine once posted at 7pm that she was enjoying a nice meal out, & again at 2am, that she was throwing her guts up. She was in fact at Pizza Express with her sister celebrating her nephew's 7th birthday, & then succumbed to a 24 hour bug that was going round.
Someone else at work saw this on FB, assumed she'd been out partying, & snitched to the boss when she wasn't in next day.
My workmate ended up having to explain, to our boss, that she couldn't sleep at 2am after honking up, & updated her status in a moment of self-pity before rushing back to the loo again.
It made her look a total wally, even if he'd believed her - & he made it clear he had his doubts.
Basically, a) never have your boss as a FB friend & b) never post anything without envisioning how it might be interpreted by unfriendly eyes c) lock down your privacy settings & d) if you must be indiscreet on FB, have an account with a false name.