Scaling that infrastructure to literally billions of simultaneous users, spread over the whole planet, is something that needs constant innovation and maintenance.

Just one example for "what needs writing".

Pedantic point: it's unlikely to be billions of simultaneous active users. They have ~757m daily active users, so that's around 32m per hour. Even with external requests from sites using FB services (which the daily active number may include - they don't seem to specify at http://newsroom.fb.com/company-info/ ) and several connections per page load, I'd be surprised if it was even close to multiple billions of simultaneous connections, nevermind users.

However, they are still dealing with damn impressive numbers :)

Most "daily active users" are likely to be logged in more than one hour (especially those using FBs Messenger) and on multiple devices (e.g. cellphone with the app and messenger, and on the desktop the "normal" website), so I'd estimate the simultaneous peak count at something like 200-500m connections.

Also, there's a bazillion requests coming in from every website which has FB Share/Recommend buttons or FB Login active...

But they just employ two DB administrators, as they outsource the rest of the MySQL maintenance to Oracle, as far as I knew? Do they really keep adding vast swathes of site content that needs an army of developers?