The "Facebook" platform is based on a set of programming interfaces along with some tools which are used to help the developers integrate the site with the open "social graph" that is collective of personal relations and other different set of information and elements like places, Facebook pages, songs, videos etc. The major such tools and programming interfaces are:
It is the core element that created the Facebook Platform, it helps the developers to read from the acquired data and write the same into the Facebook site.
The authentication helps the developers' applications in interacting smoothly with the Graph API on the behalf of all the Facebook users, providing them a single-sign on mechanism through the desktop apps, the entire web and .mobile.
Then there are Social plugins like the Activity Feed, News Feed, Like Button, Comments and Recommendations. These are for helping the developers to provide the social experiences to their users simply through a few lines of HTML.
Open Graph protocol
The Open Graph protocol help integrate the pages created by the developers into the social graph.
Facebook uses the iframes to allow the third-party developers to create their own set of applications which could be hosted separately from Facebook, but are allowed to operate within a Facebook session. They can be accessed through any user's profile as well.
The hCalendar microformat was introduced in Facebook since February 2011, to mark up the events.
So if you can integrate all these to make a website, you can try your hands in creating something as big as Facebook. But you need to keep yourself abreast and upgrade into the latest mechanisms in order to keep up the pace.