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

This question is meant to serve as a list of databases and their configurations that the major web sites use and would be a great reference for anyone thinking of scaling their web site to the size of Twitter, Facebook or even Google.

Please keep your answers to a minimum and be sure to cite any sources used.

EDIT:

Also, please bold both the web-site name and the database for easier scanning.

Solution 1:

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

Solution 2:

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/

Solution 3:

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