Getting Started with the Facebook SDK for JavaScript
The Facebook SDK for JavaScript provides a rich set of client-side functionality that:
Enables you to use the Like Button and other Social Plugins on your site.
Enables you to use Facebook Login to lower the barrier for people to sign up on your site.
Makes it easy to call into Facebook's primary API, called the Graph API.
Launch Dialogs that let people perform various actions like sharing stories.
Facilitates communication when you're building a game or an app tab on Facebook.
The SDK, social plugins and dialogs work on both desktop and mobile web browsers.