Designing an efficient social inbox requires careful consideration of data distribution and query patterns. A sharded collection can be used to support a large volume of reads and writes, but each design approach presents trade-offs in terms of efficiency and scalability. The "Fan Out on Read" method is efficient for sending messages, but less efficient for viewing inboxes due to the need to query multiple shards. In contrast, the "Fan Out on Write" method optimizes for viewing inboxes by routing queries to a single shard, but consumes more resources when sending messages. The "Fan Out on Write with Buckets" design further optimizes for viewing inboxes by bucketing messages into documents of 50 messages each, reducing random reads and index entries. This approach is generally the most scalable for social inbox applications, offering a good balance between efficiency and resource consumption.