Wasn't this kind of the intent of the original Facebook? If I remember the Social Network correctly (I get all my facts from movies loosely based on reality), Zuckerberg's first stab at the site allowed people at Harvard to rank which classmate (of two) was better looking. It wasn't exactly asking them who they wanted to fuck, marry, or kill, but it was a shallow exercise nonetheless. So this is kind of like that, only you get to play God, which is always nice.
All you need to do to play the game is go to Fuck, Marry, Kill and hit the "Connect to Facebook" button at the top. Then this is the next screen you will see next:
Next you choose a friend to play with, preferably someone who has a bunch of mutual Facebook friends with you, and then the two of you play the game for an innumerable amount of time -- thinking about sex, tying the knot, and taking lives. And the best part is: no one will be the wiser. According to the co-creator who emailed me, "Don't worry, nothing gets posted on your timeline."
All that's left is hours upon hours of carefree life-wasting and a lot of laughs (I assume).