Design an api which shows all mutual friends of a friend,how the data is stored (data model) , data structure you will use specifically at scale to support 1bn qps. This should also take care of a scenario where given an user profile the response should include common friends if any.

Design an api which takes a content and figures out which users should be able to view it, or design an api which hides contents from my friends because of the permissions In other words how to implement privacy checks.

Asked on FB onsite, 10 YOE.