Structure your system diagram clearly.
Clients at the top (iOS, Android, Web). API Gateway/Load Balancer below. Application services in the middle (feed service, ranking service, notification service). Data stores at the bottom (post DB, user graph, cache layer).
Label arrows with data flow: "POST /feed" from client to API, "get follower IDs" from feed service to social graph.
Separate read and write paths when they differ. For the News Feed, the write path (creating posts) and read path (viewing feed) use different services.