Main thing with Zuck...you gotta watch the grappling. 95% of people have never trained for rolling on the mat, so any experience you have is a big plus. He's at least done it, so that could catch people by surprise.
Outside of that, it's unlikely he has been in an actual fight in over 20 years (FB started in like 2003/2004). My guess is that he's in better shape/has more stamina than the average dude but he'd be susceptible to anyone with a decent chin that can walk through a couple of shots and maul him. Not sure he'd do much if someone gave him a clean 2-piece special.