[wtbob]

> That doesn't seem like a very good tradeoff because then it requires Signal to store/compare _more_ personally identifying information (though I guess you could make many of those things pseudo-anonymous).

Actually, it's possible to do even better that that: one can execute a private set intersection protocol so that you & each of your contacts can discover mutual contacts without revealing others.

Signal aren't required to store any personally identifying information: they choose to do so.