Database currently using by Market leader

Database currently using by Market leader

Google - Bigtable

LinkedIn.com -
Oracle (Relational Database)MySQL (Relational Database

Stack Overflow - SQL Server.

Flickr uses MySQL.

YouTube uses MySQL but they are moving to Google's BigTable.

Myspace uses SQL Server.

Wikipedia uses MySQL.


Facebook.com

Hive (Data warehouse for Hadoop, supports tables and a variant of SQL called hiveQL). Used for "simple summarization jobs, business intelligence and machine learning and many other applications"

Cassandra (Multi-dimensional, distributed key-value store). Currently used for Facebook's private messaging.