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 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.