just finished my loop two weeks ago so putting this down while it's fresh. L5 / senior SWE, applying for an infra-adjacent team.
the system design round at Meta is 45 minutes. they call it the "system design" round but it really has two parts baked into one. the first ~15 minutes is almost always a warm-up to make sure you can scope a problem. don't skip scoping even if you're anxious to jump to the solution.
what they actually care about: scale. like, Meta-scale. not "we have 10k users" scale. "design the newsfeed for 3 billion users" scale. you need to talk about sharding, fanout, cache invalidation, CDN placement. tradeoffs. they want you to make a call and defend it, not sit on the fence. "i'd use eventual consistency here because..." beats "well it depends." the data model. draw your schema early. it shows you can anchor the design.
my prompt was roughly "design a notification system." classic. but where people fail is they don't get to the interesting parts fast enough. get to: write path, read path, storage choice, how to handle delivery guarantees, and fan-out strategies for high-follower-count senders.
format they actually use: whiteboard (virtual if remote, miro or figma or sometimes just a google doc). they expect you to drive. silence is bad. keep talking.
leveling note: at L5 they expect you to propose the architecture, identify failure modes unprompted, and have a confident opinion on storage (SQL vs. NoSQL vs. a cache layer). L6 they want cross-system thinking. if you're applying L6, start the design with the constraint that the other systems already exist and your thing has to plug in.
one unexpected thing: my interviewer stopped me mid-design to say "okay the PM just changed the requirement, now you need real-time." that pivot is intentional. show you can adapt without melting down.
I didn't get to all three of my "future optimizations" but they didn't seem to care. the ones I got to were solid.
total loop: 2 coding + 1 system design + 1 behavioral (the JEDI round is separate at Meta, not baked into behavioral).