Knowing where your Jewish friend lives is fine. Having a list of every Jewish person in your zipcode (or every X) is like holding a handgrenade.

When the Ashley Madison hack happened a whole mess of blackmail was the result.

I have had someone target me in an attempt of blackmail due to a forum breach (why they thought an anime forum was good blackmail beats me...)

Your browsing habits, which is a step removed from PII, even reveals your sexuality and pregnancy status.

And, we had an attack on Jewish people in the last month. We have not had anyone Democrat/Republican hunting.