To say that PHP/MySQL is just a "part of Facebook's stack" is laughable.
They are the core components of Facebook. Normal people understand that the characteristics of Facebook's architecture is unique to just Facebook. They can get away with sharding/colocating data that nobody else can. The rest of us have a tonne of integrated data that requires complex joins (whether at the application or database layer).
They are edge components of Facebook. Just from a brief interaction with FB recruiters, I learned they use a lot of Vertica in their back-office. Please don't propose that they are using MySQL for their main business when it's only powering app nodes which are just POPs fed by their real (internal) services. Approximately speaking.
They are the core components of Facebook. Normal people understand that the characteristics of Facebook's architecture is unique to just Facebook. They can get away with sharding/colocating data that nobody else can. The rest of us have a tonne of integrated data that requires complex joins (whether at the application or database layer).