Facebook is growing 3% per week currently (6th most trafficked site in the U.S.). I guess it will not be long before Facebook would catch Myspace traffic and leave it behind. A week ago (05/24/2007) Facebook launched Facebook Platform - giving away the Facebook API and the ability to integrate 3rd parties applications into their website.
I'm a bit late on it since a bunch of companies including Microsoft, Amazon, Slide, etc had developed prototypes and even production-level applications to integrate.
I've made a bit of reverse engineering through Facebook business entities and created a class diagram - on the screenshot below (clickable)
Please note - this is not object relational diagram (entity diagram).
Connected on this diagram means some type has a field of some other type.
So photos are not related with tag and album. Each of them can be independent.
Applications are not included in the Facebook API currently as far as I know...
More than 2 years ago I sort of reversed-engineered and created a Facebook Object-Oriented Diagram which demonstrates relationships of objects and entities used by Facebook. To my surprise this diagram was later used as example by many Facebook development related groups and linked to in blogs. Today Twitter is a new kid on the block and now [ ... ]