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I still don't get your point here, I know about the option, but if you just give them the phone number, how they can get access to the phone book? And surely its better to not give them the phone number. But anyways, even Steemit asks you for your phone number (if you do not pay for your account) so they could somehow connect you to the real person, not just "someone, who might be a fake-account".
And LinkedIn is asking for your email address and password, to reach more of your friends, this is even more absurd, well more absurd is if someone give them the password for any stupid reason like this.

Facebook also asks for your E-mail to be connected. What they are looking for is your address book. As for contacts what they do is ask permission to access the contacts on your phone. Once you give the OK, FB copies all contacts and keep them in the database.

Facebook have your email address, but the connection is also optional. If anyone allows them to enter phone book and email address book, that is very irresponsible and scary at the same time. As if it is not enough that Google knows almost everything about you. Surely the protection of private data is important and will be more important in the future.