Facebook Connect is the next iteration of Facebook Platform that allows users to “connect” their Facebook identity, friends and privacy to any site. This enables third party websites to implement and offer even more features of Facebook Platform off of Facebook – similar to features available to third party applications today on Facebook.
Facebook has lauched Facebook Connect with 24 partners initially. Those partners include CBS, CNET, CollegeHumor, Disney-ABC Television Group, Evite, Flock, Hulu, Kongregate, Loopt, Radar, Seesmic, Red Bull, Socialthing!, The Insider, StumbleUpon, Twitter, Uber, Vimeo, Amiando, Plaxo, and Xobni.

The OpenSocial API is a set of common APIs for building social applications on many websites. There are two ways to access the OpenSocial API: client-side using the JavaScript API, and server-side using REST or RPC protocols.
JavaScript API