That Zuck wrote the first version of thefacebook.com in PHP, the language he was the most productive at?
That the initial team didn't rewrite Facebook in Python/Perl/Ruby/Haskell during the fast growth phase? If you have ever experienced the growth phase, you understand how ludicrous the idea of rewrite would be. I've personally experienced and heard only horror stories about rewrites. We underestimate how much hidden wisdom a production code base has and that the messiness is often there for good reasons.
That Zuck wrote the first version of thefacebook.com in PHP, the language he was the most productive at?
That the initial team didn't rewrite Facebook in Python/Perl/Ruby/Haskell during the fast growth phase? If you have ever experienced the growth phase, you understand how ludicrous the idea of rewrite would be. I've personally experienced and heard only horror stories about rewrites. We underestimate how much hidden wisdom a production code base has and that the messiness is often there for good reasons.