The News Feed is Meta's signature system design question. Here's the high-level architecture:
Write path: User creates a post. Post goes to a post store. A fan-out service pushes the post ID to followers' feed caches.
Read path: User opens app. Feed service reads from the pre-computed cache. A ranking service orders the posts by relevance.
The fan-out decision: For users with few followers (most people), fan-out on write. Pre-compute their followers' feeds. For celebrities with millions of followers, fan-out on read. Compute their posts at read time to avoid writing to millions of caches.