I've worked at two of "FAANG" and know enough people at two of the others (all but Netflix). The vast majority of Americans at all those companies went to mediocre public universities. (I don't know enough about foreign educational credentials to judge whether that's true of non-Americans).

The notion that FB/Amazon hire primarily from the top 5 universities is so totally unrelated to reality that I feel like I'm watching people discuss the consequences of the fact that the earth is flat.