A more down-to-earth example: Evernote is (almost entirely) on MySQL. Since most of the stuff is "yours", it's fairly easy to shard partitioning by user. And th

A more down-to-earth example: Evernote is (almost entirely) on MySQL. Since most of the stuff is "yours", it's fairly easy to shard partitioning by user. And the global stuff which is not that much (users/auth/etc) is stored in beefy replicated instances. Newer features are being built on other data stores, and some things are being migrated away, but the core stuff is still in MySQL.