Facebook has its own API. It became popular with launch of applications.
There are many other networking sites like Orkut, MySpace, hi5, linkedin etc.
What if all creates their own API? Developer will have to create different application for each platform.
What an idea…
What is OpenSocial?
Journey of OpenSocial
Launched on November 1, 2007 with initial version 0.5
Initially supported by Friendster, hi5, MySpace, Ning, orkut and many more.
At launch Flixster, FotoFlexer, iLike, Newsgator, RockYou, Slide, Theikos, and VirtualTourist presented applications.