The Feed Problem
Generating feeds for millions of users is expensive. Querying "followers of following" in real-time kills databases.
Graph Databases
Use Neo4j or Amazon Neptune for social relationships. Relational DBs struggle with "friends of friends" queries at scale.
Media Storage and CDNs
Store metadata in DB, binary files in Object Storage (S3). Use aggressive CDNs (Cloudflare/CloudFront) to serve images locally.
Caching Strategies
Redis is your best friend. Cache user profiles and popular feeds. The "Thundering Herd" problem must be managed.
Fan-out on Write vs Read
For celebrities (Justin Bieber), fan-out on write (push to all followers) is impossible. Use hybrid approaches: Push for normal users, Pull for celebs.
Real-time Updates
Websockets for notifications. But don't open a socket for everything. Use push notifications for background updates.
Content Moderation AI
You cannot moderate manually at scale. Implement AI text/image analysis to flag toxicity and NSFW content instantly.