Facebook now asks users in India to give their official biometric ID linkage.

Being used by Facebook was harmful already. This makes it more harmful because it enables Facebook's data about people to be linked easily with many other databases also indexed by national ID numbers.

This is an example of why a national ID number is harmful. Once everyone has that number, it becomes natural for every business or organization which has a database of people to index it by national ID number — and presto! they are all cross-linked.