Facebook Feed Ranking: Pick at Most Five Signals That Reveal a User's Closest Friend

You are a product manager at Facebook. The team wants to identify, for every user, which person is that user's best friend, so that the friend's posts can be ranked higher in the user's News Feed. Using no more than five product signals or metrics, explain how you would determine who a user's best friend is and why you chose each one. ```hint Closeness is not similarity Two people can share a school, a hometown and many interests without being close. Favor signals that show the two people actively choosing each other, in both directions and over time. ``` ### Clarifying Questions - Should partners and family members count as possible best friends, or only friends who are not relatives? - Must the best friend already be on the user's friend list, or can it be someone the user follows or messages without being connected? - Is the goal a single best friend per user, or a ranked group of closest friends whose posts all get a boost? - May private messaging activity be used as a signal, counted but never read, given users' privacy expectations? ### What a Strong Answer Covers - An operational definition of "best friend" that a system can measure, tied to the Feed goal. - At most five signals that draw on different kinds of evidence, each with a concrete way to measure it and a known weakness. - Reasoned exclusion of signals that are weak proxies for closeness or that raise privacy or fairness concerns. - How the signals combine into a ranking, and how the result is validated against ground truth and in an experiment with guardrails. - Edge cases such as one-sided relationships, new users, breakups and privacy. ### Follow-up Questions - Messaging between a user and their predicted best friend suddenly stops after a falling-out. How should the score react, and how quickly? - How would you tell whether the boost made feeds better for users rather than only more engaging? - Two friends have nearly identical scores. Does it matter which one is called the best friend, and how would you design the boost in that case?