If Mark was a hacker of some kind, he was a growth hacker. That's what Facebook was (in)famous for in its early days - implementing various little psychological tricks that drove adoption. A lot of that came from Mark (initially, pretty much all of it).

I'm not aware of any evidence that he was an exceptional engineer. So what? Why would he even care about being that? As long as you can hire engineers that are better than you, you don't need to be a great one to grow a social network. We're not talking about AI here.