That sounds awesome, but if it’s network-wide, then marketers (who are also in the network) can also use it. Even if it’s some sort of cryptographically-secure

That sounds awesome, but if it’s network-wide, then marketers (who are also in the network) can also use it. Even if it’s some sort of cryptographically-secure bitstring, all it would take is for you or one of your contacts to leak it once. I don’t think it’s possible to have a persistent, global, private identifier: they seem mutually-exclusive.