If that's his way of breaking up then he is a pussy. Immediately confront him on this, and be forceful when you do so. As a straight guy, I would expect that kind of reaction to call me out. Further bullshit is possible, like "it was a mistake", but if the guy is a facebook veteran then you know he's full of shit.
I agree with the first one my bf pg on facebook had done the same thing...well it was a mistake he hadn't even been on in 2 days dont jump to conclusions...ask him dont get any attitude just ask and if he did...GET REVENGE FOR WASTING UR TIME WITH A PUSSY!!