Design the News Feed

This is the marquee product question in LinkedIn's design loop. The home screen is what a member sees the moment they open the app, and the catch that makes it LinkedIn and not a generic timeline is that the order is earned, not chronological. A two-day-old post from a close colleague that is getting comments can rank above a fresh post from a company you barely follow. So the design has to do two hard things at once: gather the right candidates from a member's slice of a billion-member graph, then rank them well.

Scale sets the shape but is not where the round is won, so put the numbers down and move on. LinkedIn is north of 1 billion registered members. Only a fraction are active on a given day. Call it a few hundred million members opening the feed, several times each. That lands feed builds in the rough range of a billion a day: roughly 12,000 a second on average, and several times that at the weekday-morning peak. Posting is far rarer than reading. Most members lurk. A working estimate is on the order of 10 million original posts a day, with a much larger volume of reactions, comments, and reshares riding on top. That read-to-write skew, hundreds of reads for every post, is the single fact every later decision leans on.

The LinkedIn angle

This is product architecture, and the interviewer ships the exact system you are sketching. A generic "design Twitter" answer goes nowhere. What does land is reasoning about the professional graph: a member's first-degree connections are mutual and trusted, a followed creator is one-directional, and a company page fans out to millions. Say early that the feed is read-heavy and tolerates a few seconds of staleness, then spend your time on the two parts they actually probe, fan-out under a high-follower creator and ranking. Treat this as a pure scale problem and never mention ranking, and you have answered half the question.