The part I think most people miss is that how "good" Zuckerberg is/was matters less than the resources he had access to. Luck/privilege account for SO much and that's true of all most all the billion $ startups.
There are a ton more to keep going on.
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.
2. Being white
3. Being cis
4. Being straight
5. Being born in the US