Allowing Access to Data Only When You Share Your Similar Data

I've noticed websites, apps providing privileged data, only if you are willing to share similar data from your side, or may be by paying to see others' information but protecting yours.

Examples:
  • Truecaller (a collaborative phone directory to find people from all over the world through name and phone number lookup) allows you to view the name of an unknown number only if you share your address book with them.
  • Orkut, a social network site had a feature for showing profile visitors, but those will only be displayed only if you allow your visiting someone's profile to be logged in their history.
  • WhatsApp, an inter-mobile-ecosystems communication app which is just like text (SMS) facility allows you to see the last time a person was active on WhatsApp, but only if you allow others to see your last active date and time.