TrueCaller is Facebook's face recognition for phone numbers. They are building shadow profiles using some user-valuable bait feature. For TrueCaller, most of its value is in fighting spam. So, unlike Facebook, it's not only users who compromise privacy of others, it's spammers who steal our privacy. We should examine spam fighting solutions for email, IM and others, they might be involved in the same business model.…
This is very good analogy - in fact I think that it's even more dangerous because when it comes to Facebook people are slightly more aware about privacy-related issues. I'm pretty sure that TrueCaller extensively profiles users, links all possible connections, gathers data - Facebook might not know that one visited certain doctor or that one has some medical conditions, TrueCaller? Who knows. It is possible to gather a lot of data and build detailed profile just by linking who calls who.
Additionally according to HN comments [0] one doesn't just simply "opt out", truly terrifying