Design a Social Media Feed Ranking System

Build a **social media feed ranking system**.

Include:
1. Feed fan-out vs fan-in evaluation
2. Feed generation pipeline
3. Ranking models (engagement, freshness, personalization)
4. Spam filtering
5. Caching strategies per user segment
6. Pagination and infinite scroll design
7. Write amplification management
8. Hot content detection
9. A/B testing architecture
10. Multiregion replication considerations