I have been following the ascent of Facebook, just like everyone else in the Bay Area that likes technology. The advent of a company that does so much so well is astonishing, especially after considering all the competitors that didn't do so well.
In particular, Facebook opened up their own application to third party app developers. They provided a series of APIs that allow geeks to create add-ons that use the Facebook database and integrate into its user interfaces and communication mechanisms.
Most of the apps that came out early on were simple dissemination gadgets. It was about "infecting" your friends and creating as wide a circle as possible. This is in direct response to the concept of the social graph that is so close to the company's founder.