This is something I actually have thought about. A few years ago one of my friends died and his Facebook page was bombarded with messages of consolation or people asking what had happened. It seemed odd because it was not like he was going to answer them back!
In the end, I'm guessing someone left their Facebook page open at work and got sabotaged by a co-worker. I've seen it a dozen times. There's also the slim possibility that it's real, and THAT is unreal.