I used to work at Meebo, which hosts a large CouchDB system , but when it came time to choose a solution for Instagram we went with PGSQL. There was a lot I really liked about Couch, especially for the analytics system we built at Meebo (where the map/reduce views worked great), but I wouldn't classify it as a low-Ops-burden technology--like any newer db solution, there are some rough edges and more 'unknowns' at scale.
Also, we'd still have to write the middleware to assign data to shards and fetch data from shards in our system (unless we used something like BigCouch), so having a more tried-and-tested solution that we already understood well was more appealing.
Also, we'd still have to write the middleware to assign data to shards and fetch data from shards in our system (unless we used something like BigCouch), so having a more tried-and-tested solution that we already understood well was more appealing.