Web company databases

Almost none of them use Oracle, so what do they do?

AWS customers have reduced costs by 90% by migrating from Oracle to AWS, according to Amazon. (Source).

 
Their solution
Google Percolator for search engine. Successor to BigTable, another solution created by Google.
Spanner. BD NewSQL, distributed and replicated in real time, for GMail and others.
Facebook HBase. Derived from BigTable and is part of Hadoop, a toolkit.
MySQL. For general functions. TAO oversize for social graph.
Haystack. For photos.
Hive. Adds data analysis to Hadoop.
Diagram: Facebook infrastructure when working with databases. (Source, Malta Schwarzkopf).
Twitter Cassandra on statistics and geolocation .
MySQL. Manager fork, for users and tweets. The source code is available on GitHub.
FlockDB. Graphical database of social connections. The source code is also on GitHub.
HBase и Hadoop. Probably to replace MySQL.
Wikipedia MariaDB. Shared wiki clusters. Programmed in PHP and JavaScript (30%).
Yahoo Modified MySQL. The company had a record for the size of SQL tables .
PostgreSQL, part of whose query has been configured .
eBay Headup with Kylin, Data Mass Search System. eBay used Oracle.
Digg DynamoDB, which is a NoSQL solution provided by Amazon AWS, its hoster.
Netflix Cassandra.
Skype PostgreSQL. Used with PHP and other scripting languages.
Youtube MySQL + Speed ​ ​ to adapt to the cloud .
Uber Ported from PostgreSQL to MySQL (Percona) + Schemaless, making it NoSQL.
Cassandra, Hadoop, MongoDB and Redis under Node.js are also used (Source)

This is just a selection of the most famous companies. Many others use PostgreSQL, MariaDB (replacing MySQL), HBase, and Hadoop tools.
Even though Cassandra was left to Facebook, which created it, the software is used by a variety of other companies with different needs.
Only Amazon used Oracle, which gradually replaced other databases, partly Aurora, its own solution. The decision was already made back in 2014 and on October 15, 2019 announced that the last Oracle database was finally deleted.
Oracle CEO Larry Ellison on October 26, 2018 said:

Amazon runs entirely on Oracle. They were unable to migrate to AWS as it is not effective enough.

It does take a long, long time to migrate a database, but that's not because the news isn't powerful enough.

It can be seen that companies use MariaDB (possibly in clusters) and PostgreSQL for structured data. But they also use them in combination with other tools for network data.
Uber's experience has shown that PostgreSQL is not suitable for bulk row updates and that MySQL or derivatives are faster until you change the primary key. However, Postgre provides more options in other uses.
Moreover, other types of databases are better suited for a wide variety of web applications: NoSQL tables, documents, relationship graphs, key-values, etc.

See also...