Facebook recommends patients with a psychiatrist's number on phone friend one another—outing them to each other. 

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With more and more apps FB gets more and more opportunities to collect metadata (FB/Messenger/Whatsapp/Instagram/Lifestage/etc).
If phone no is a source of privacy violations by Facebook you'll withhold it, but lose 2FA. You're now more hackable. Not a good trade-off.
If WhatsApp—availability & security is unmatched—is sharing metadata with Facebook and that exposes you, many will switch to insecure apps.
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