Design a people-you-may-know friend recommendation system

Design the "people you may know" feature of a large social network: each user sees people they are not connected to yet but probably know, and can send a friend request or dismiss a suggestion.

Cover how candidates are generated from the friendship graph (friends of friends and other signals), how they are scored and stored, how often lists are refreshed, how dismissals and new friendships take effect, and how the system scales to billions of edges.