Facebook's implementation would still be constant time for short strings, because there's a constant which bounds the runtime.

Though I hear that the definition of big-O notation has shifted a bit in Silicon Valley these days so maybe that answer would get me in trouble in an interview.

It's true that big-O notation only concerns behavior with large N, but it's a bit disingenuous to say that the loop executes a constant number of times -- by that argument, you could say that if you implemented size() by strlen() it's O(1) because the string must be less than 2^64 bytes long on a 64-bit machine.

So I can see why someone would claim that implementing size() via strlen() "only" for small strings shouldn't be considered O(1), because strlen() is O(n) and within that class of strings the runtime is increasing as the length increases.

I think it’s a bit more disingenuous to compare the magnitudes of 2^64 and 23 for the sake of argument, as if 2^64 isn’t practically asymptotic.

Even if the standard required O(1) it would conform because the loop is bounded (<= 23), it’s not O(n)

To be O(N), the time required for the lookup would have to continue to grow indefinitely. This has an upper bound.