Here is summary per bullet on the software architecture of facebook. The main site is indicated at the end of the blog.
– comments, likes, friends are by nodes using a mysql database
– has 2 layers of cache ( leader cache and sub caches under it )
– bypass access to data directly to database if caches fail, or even from master db systems
– as all other large systems, only links to the photos are stored in the db, which points to a another photo storage.
– as explained here, this TAO system is only the site rendering system. For other processes not handled by TAO, it is on another system