I believe Mark Zuckerberg simply used the technology he knew and took it from there. That's fine. I probably would have done the same thing.
But many people are making an ideology out of this, arguing that not giving a shit about performance is always the right choice initially because that's how you grow fast enough to be able to retool later.
I think this is based assumptions that are no longer true.
In the early 2000s, mainstream programming languages and runtimes were either fast and low productivity or slow and high productivity (Exceptions such as Pascal/Delphi did exist but they were not mainstream). And the cost of scaling up was prohibitive compared to scaling out.
Today, you can choose any fast high productivity language/runtime and go very far mostly scaling up.