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

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If WhatsApp—availability & security is unmatched—is sharing metadata with Facebook and that exposes you, many will switch to insecure apps.
As for "use Signal". You can use Signal all you want. I do. Ordinary people—doctors' patients, lawyers' clients—are very hard to convert.
you don't need phone no for app generated 2fa, and app generated passcodes are safer than sms 2fa https://www.wired.com/2016/06/hey-stop-using-texts-two-factor-authentication/ …
I know. Very hard to convince people of that.
FB suggested I friend a woman I called about cleaning my mom’s house. I wondered how that happened!
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