Actually the theory says that it is for everybody. The idea is that it grows exponentially. So even though you might only shake hands with a few hundred people, these will each shake hands with a few hundred people. There are of course some overlap, but still the transitive closure grows extremely fast.
That said, I suppose you are right. There are very few indigenous peoples left who doesn't communicate with the rest of the world. But you might be able to find some. And then it's not possible. Also some religious subgroups are very closed among them selves. So I guess you are right. It does hold true for almost any two persons you choose though.
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