Facebook's main code base is more than 9.2M lines of PHP code (excluding all supporting backend systems written over time). It's estimated the company owns and operates more than 60,000 servers. As of July 2010, the site received 100 billion hits, held more than 50 billion photos, cached 3 trillion objects, and accumulated 130 TB of logs a day. 

Facebook is the most complex distributed software system ever created.

And the Space Shuttle is the most complex machine ever built. It had 2.5 million parts, including nearly 370 kilometres of wire and more than 1,060 plumbing valves and connections, 1,440 circuit breakers, and 27,000 insulating tiles and thermal blankets.

Facebook might not be the largest software project ever built — in fact WIndows 7 probably has a larger code base — but when it comes to daily usage levels, number of users and operational complexity, Facebook wins hands down. 

Facebook is the Space Shuttle of software.

And like the Space Shuttle, it's a massive endeavour that requires thousands of people to support, maintain and improve it. It cannot fail.

But just as the Space Shuttle has been retired, so will Facebook be placed on the funeral boat and shoved out to sea.

Something better will replace it, just as the Space Shuttle is being replaced by smaller, leaner craft.

What will replace Facebook?