The fact that address-book upload should be opt-in is obvious, and was stated so many times here it was boring. But, there's also the other side: me, and quite a few of people I know, have good reasons to have an opt-out from being discoverable this way. If someone knows my email address, let them send me an email with an invitation code. They shouldn't even know I'm signed up until I accept.

Though I also don't really think it's something private companies should solve. Now, I can of course avoid services that let me be too easily findable, but the proper solution is to make said opt-out required by law. Otherwise it's just not beneficial for the company to provide it.