High Scalability – High Scalability – Facebook’s Memcached Multiget Hole: More machines != More Capacity
also see http://dormando.livejournal.com/521163.html Describing some problems with performance/scalability you may have if you partition/shard your data by hash. ( that's what memcached does.) You may hit the cpu bound limit from the amount of requests that few very popular keys receive. Currently, there is now common way around this (perhaps because very very few sites scale to the point when this becomes an issue )