MySQL refugees = those who ran to MariaDB when MySQL was bought by 'Orrible and now need another new home. Accidentally, PostgreSQL has grown support for some of MySQL on recent versions.
There were so many web apps written in the early 00s on the LAMP stack, including Facebook. And that’s not counting the tiny internal applications that so many businesses have that use MySQL/MariaDB. Because these are business critical applications, they pay Oracle/MariaDB for support.