Justin Laing from the MerchantOS group submitted a link to a Facebook tutorial that shows hos to get connected to one of the most popular social networking applications via PHP.
I will cover the basics of using the Facebook PHP library and how to get your application started, including: installing the Facebook Developer application, downloading The Facebook PHP client library, creating your application, Hello Facebook! Example, FBML - Facebook Markup Language and using the Facebook API.
He starts with his download of the API, including the creation of the application profile and of an API key you'll use to interact with Facebook's API. The simple "Hello" script is next - one that echos out a "Hello [username]! Welcome to my first application!" message to the page's viewers. To help customize the page, he also shows how to make the page "addable" for other users versus just there on login.