Don't get me wrong, I would generally choose Postgres over MySQL for an RDBMS with replication requirements these days, but I'm not sure I would have made that same descion a few years ago.
There are valid reasons that long established companies such as Google, Twitter, Facebook and countless others chose MySQL as their primary data store.
And with that in mind, I would take the slow to deliver replication options postgresql supplies in base over the previous third-party options which where comparable to the built-in options in mysql... and I'd take it over mysql's built-in options.
and I've used both postgresql and mysql in highly transactional/reliable production environments for over 5 years.
[I know there's different opinions but I'd rather have a fully open database if I have a choice. For the record I default to PostgreSQL]
I think you're misrepresenting things here. While all do use MySQL for some specific tasks, it's clear that all also have many other central datastores which are not MySQL. In Google's case this is a gross overstatement.