Today I decided to give in to all the peer pressure and install WhatsApp on my phone. Of course, they want me to accept their terms of service and privacy policy, so I go and read them (yeah, I'm weird like that...), and stumble upon this section:
WhatsApp ToS said:
Address Book. You provide us, all in accordance with applicable laws, the phone numbers of WhatsApp users and your other contacts in your mobile address book on a regular basis, including for both the users of our Services and your other contacts.
My first thought is: "how do I have the right to do so?" and as it turns out: I don't!1.
In essence, the only way for me to legally install WhatsApp would be to get explicit permission from all people in my address book to share their phone number with WhatsApp. The chance of this happening is, in a first, rough estimate, indistinguishable from zero.
This also means that all my friends who have my phone number in their address book, and have WhatsApp installed on their phones, have done so illegally and technically I could sue them for violation of my privacy rights.
1 This lawsuit was in Germany, where I don't live, and before the GDPR came into effect; but do I live in a country where the GDPR applies, and it seems unlikely that the GDPR has weaker provisions in this regard than what Germany had before, so chances are this would now apply here, too.