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Select * from fb_users join wp_users on wp_users.id = fb_users.phone_number A really basic query that will give you all the fb users who have listed their phone number. Also i just realized while typing this, i think fb started allowing logging in using your phone number around the time they bought whatsapp. So even the profiles that didn't give their phone number to fb, they could get it via this logging system. ( I…
FB does not require phone numbers so you are just decorating with data that most users do not provide to FB. Not the same thing. Not saying FB could not join the tables given how much data they have, but it is not nearly as simple as this.
And you're basing this on... what? Facebook nags you EVERY TIME you login for your phone number. EVERY TIME. I can tell you every last one of my friends who is non-technical has put in their phone number simply to get rid of the nag message. While my evidence is anecdotal, it flies in the face of yours. So I assume you have a citation for your "most users" comment.