Oracle might be popular, but few places can afford, or justify paying for it. MySQL is mostly popular because everyone was busy cargo culting based on what Facebook and YouTube were doing instead of evaluating the choices on their merits. Luckily most of those people have moved on to cargo culting NoSQL choices.
If you want a solid open-source database, especially one that has a future, learn Postgres.
So I guess if all of those clueless engineers at the world's most trafficked websites would have just listened to you they would never have picked MySQL. Have you ever thought of letting them know that they should switch over to PostgreSQL ?
The engineers and Facebook might have selected MySQL on merits. Or they might be stuck with trying to do the best with legacy choices. I don't know what the best solution is for them today, and it might not be either Postgres or MySQL.
If however you need to make a choice today for what database to use on a new project, then you should evaluate the choices instead of basing your decision on what might have been best for Facebook 8 years ago.
I'm not going to get into the technical differences here, because even if MySQL and Postgres were technically equivalent (and they aren't but I'll let you do your own research), I simply don't trust Oracle with the future of MySQL.
If you want a solid open-source database, especially one that has a future, learn Postgres.