You've practiced medium-complexity designs.
Twitter: Hybrid fanout (write for normal users, read for celebrities). Timeline caching. Search as separate system.
WhatsApp: Persistent WebSocket connections. Message queues for delivery. Store-and-forward for offline users.
Notifications: Multi-channel routing. User preferences. Rate limiting to prevent spam.
These patterns recur across social and messaging systems. Next phase will cover media streaming and e-commerce designs.