MySQL is what we started with, as time has gone by, we've migrated more and more data to other data stores. Redis and mongo solve two different problems for us

MySQL is what we started with, as time has gone by, we've migrated more and more data to other data stores. Redis and mongo solve two different problems for us. Redis is great for the social graph, which is how we've started experimenting with it, whereas mongo is best for just about everything else that doesn't hold the data in MySQL.