We're not necessarily talking about sharing contact information; all we're talking about here is an acknowledgement of existence.
Given that context, due to the nature of the problem, rate limiting is by definition the -only- solution to the problem (if I'm understanding the problem correctly.)
You do have a point that perhaps the question we should be asking should be 'how do we detect mutual connections,' but that's another story altogether.
Under what circumstances do I want someone who I have not even enough interest in to put in my phone's dialer to be able to ping my phone number and get my name back?
The question of whether services should even be DOING this kind of open book "does this phone number have a user attached and what is their name?" query is more fundamental than any other question, imo. It's not another story, it's a skipped first story.
Given that context, due to the nature of the problem, rate limiting is by definition the -only- solution to the problem (if I'm understanding the problem correctly.)
You do have a point that perhaps the question we should be asking should be 'how do we detect mutual connections,' but that's another story altogether.