1) Application connection pooling won't scale. In a scenario like FourSquare, there is likely a 4:1 ratio of app server to DB shard server. Further, connections don't necessarily equal load.

2) This sounds like a great way to create data inconsistencies, unless you've got very tight constraints on your database, which is impossible in a sharded scenario.

I agree though, that ultimately they should have had some way to "fail whale" instead of getting overloaded.