At 10×: the database write path is the bottleneck. Sharding by user ID or content ID distributes writes. The read path moves toward dedicated read replicas. Consistency guarantees become harder to maintain across shards.
At 100×: data volume requires tiered storage — hot data in fast databases, warm data in object storage, cold data archived. The query model changes: not all historical data can be served at the same latency target as recent data.