If the data was sharded by user ids and some ranges of user ids were more active than other user ids, why not shard by a hash of the user id. In that case all

If the data was sharded by user ids and some ranges of user ids were more active than other user ids, why not shard by a hash of the user id. In that case all the data for a given user id would live in a single shard, but active ranges of users would be broken up across shards. If any single user is creating too much data for a single shard, you've got other problems.