FB has thousands of employees who write code. I highly doubt many of them are writing sorting algos from scratch. They should know which sorting solves the problem and use an appropriate library.

The same applies to most other low level problems. They should understand the problem and the best practice to solve it. In most cases writing low level solutions from scratch is not the best practice.

FB is a mature company now with a slowed growth rate. They've been dealing with scale and performance for long enough now that it shouldn't be necessary to write all of these things from scratch.