D. He had parents who indulged his interest in programming early on and hired a software engineer to privately tutor him E. Happened to have a group of friends (including Adam D'Angelo, later Facebook employee) at his VERY expensive private high school that were also great programmers by that point, likely helping each other become better.

I'm not saying the first versions of Facebook were ingenious feats of engineering, but Mark easily may have never learned to build things. Considering everything else mentioned in this thread, we can reasonably conclude that a lot of went very right for him at many points along the way.