Facebook has swerved from open source MySQL 5.6 using too much customization. They also do not use InnoDB which is actually well tuned for NVMe storage systems. The customizations make it far more costly to migrate to MySQL 8 and PHP 8 etc. Facebook now has a swarm of developers working for the last year trying to migrate to MySQL 8.
Facebook,. Twitter and even Hardcore Games all use MySQL. Here the site uses the stock databases so updates are trivial. Customized themes and plugins are provided by WordPress which lines up fine with the PHP programming language and the SQL database.
WordPress allows developers to use the database for tools like the WP Users Online tool. The tables are small and they are able to identify the search engines fairly easily. The malicious users are now banned. SQL handles all the needs of the site easily. PHP can handle any custom container needed.
Twitter seems to be slightly better run as the site does not seem to bog down like Facebook does.