

OpenSocial is a set of common application programming interfaces (APIs) for web-based social network applications, developed by Google along with MySpace and a number of other social networks. It was released November 1, 2007. Applications implementing the OpenSocial APIs will be interoperable with any social network system that supports them, including features on sites such as Hi5.com, MySpace, orkut, Netlog, Sonico.com, Friendster, Ning and Yahoo!-Wikipedia.com
Here is am interesting presentation on Open Social
Sounds great right? So why is Facebook is denying the love? Facebook COO Owen Van Natta responded by saying his company was "evaluating" OpenSocial. "Standards which reduce friction for developers are good," he said. "Facebook is going to embrace standards which enhance those things." However, "privacy and control of security is really important. If that is going to be undermined in any way, that is something we can't do," he added.-SeekingAlpha.com
Seems to me that the other reason is that they have Facebook Connect. Facebook Connect is a single sign-on service that competes with OpenID, the service enables Facebook users to login to affiliated sites using their Facebook account and share information from such sites with their Facebook friends.-Wikipedia.com
Come on Facebook-can't we all just get along?