But the engineers at FB do need to be able to write sorting algorithms from scratch rather than just have knowledge of sorting. The reason is that the engineers have to be reasonably fungible so that they can be easily moved between projects as projects are started and stopped. So they could easily be tomorrow thrown into an environment where a sorting algorithm is needed, they are the ones responsible (so they'll need to build it, not just have a high-level idea of how to do it), and at FB scale every bit of squandered efficiency is very expensive (if their photo search algorithm, let's say, takes 5% more memory than it needs to, that will end up costing the company much more than their salary).