I'm doing a technology assessment of using hibernate shards to implement a social network (eg: facebook-like web application).

Hibernate shards seems to work best if the application can be cleanly split into shards.
But a user on a social network will live in one shard and have many friends on many different shards.
Loading those friend objects from all those different shards seems to defeat the purpose and benefits of shards.
Any advice on using hibernate shards to implement a scalable social network architecture?

It seems like the architecture for a sharded social network application will have limited copies of the global user base in each shard. This way, each shard can access all 'friends' inside the shard. But this would mean storing and updating all that friend data in each shard. Any other suggestions?

I would not map the association and only store the friend ids. You can then load friends manually.