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A huge amount of infrastructure is needed for Facebook's scale. People have to create, manage, and maintain that infrastructure. And that's ignoring the multitude of complex user-facing products Facebook has (the number of which has grown significantly over the last few years).

To get a sense of what kind of work is done at facebook, please visit their careers page:

https://www.facebook.com/careers/teams/

I don't think people should have downvoted you for this, it is a reasonable question to ask! Consider that their core application's user facing functionality is not the most complex in the world, but they deploy it for over 1.5 billion people. There are numerous complications and logistics involved with doing that. Think of it this way, how many organizations can reach billions of people with just 12,000? We already take for granted that technology can do incredible things like this that haven't been possible in the past. One other thing is that facebook has contributed to some awesome open source projects and their technology blog has some interesting insights on this https://code.facebook.com/posts/