That is a bit simplistic and is missing a bunch of things. First the numerous C++ and Java backend services that do most of the heavy lifting like producing th

That is a bit simplistic and is missing a bunch of things. First the numerous C++ and Java backend services that do most of the heavy lifting like producing the feed, doing searches (of various types, both from what people type in, and internal ones), sending and receiving messages, spam and fake account detection, &c. The PHP portion acts as a front-end to these services. TAO is the graph caching system that has largely replaced memcache at Facebook. Just on the traffic side, there is the global traffic management system (for dynamic and for static content), different types of load balancers, CDN-specific caching software, image and video transcoding/encoding, &c. And that's just the stuff involved in the production path - there are all sorts of behind-the-scenes things that do async work of various types - either as soon as possible (video encoding, updating) or on a daily or whatever cadence to update infrequently-changing things.