What databases do the World Wide Web's biggest sites run on? [closed]

Facebook.com

• MySQL with MyRocks. Used to store user info and social activities such as likes, comments, and shares.
• 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.

Currently running 610 (soon to be 1000) Hadoop nodes in a single cluster with Hive datastore. Both Hive and Cassandra have been open-sourced by Facebook.

Facebook stats:

• More than 200 million active users
• More than 100 million users log on to Facebook at least once each day
• More than 30 million users update their statuses at least once each day
• Average user has 120 friends on the site

Sources:

• http://www.dbms2.com/2009/05/11/facebook-hadoop-and-hive/
• http://www.facebook.com/note.php?note_id=89508453919
• http://www.facebook.com/press/info.php?statistics
• http://hadoop.apache.org/hive/
• http://wiki.apache.org/hadoop/Hive/Design
• http://www.facebook.com/note.php?note_id=24413138919
• https://code.facebook.com/posts/190251048047090/myrocks-a-space-and-write-optimized-mysql-database

Stack Overflow - SQL Server.

Jeff Atwood wrote a nice blog post on this

https://blog.stackoverflow.com/2008/09/what-was-stack-overflow-built-with/

LinkedIn.com

• Oracle (Relational Database)
• MySQL (Relational Database)

Databases replicated on multiple servers for high availability. Each specific Service uses its own domain-specific DB.

LinkedIn stats:

• 22 million members
• 4+ million unique visitors/month
• 40 million page views/day
• 2 million searches/day

Sources:

• http://hurvitz.org/blog/2008/06/linkedin-architecture/