To author: For how long could you have avoided the need to chard if you would have off-loaded all the read-only queries to multiple hot-standby slaves instead? Your scaling story is almost identical to ours, same pg versions on same years, DRBD, etc. But we haven't split the database into multiple chards yet. I'm hoping to post-pone it for at least a year by off-loading read-only queries to a lot of synchronous slaves.
I would imagine given their product they have a very write heavy work load and the read load, while non-zero, isn't a major concern. In typical web app environments however, you can definitely increase your "time to sharding" with read slaves and increased caching.
