If you're running a social site, you can let users take their profiles and friends with them to other sites and applications by supporting the OpenSocial REST or RPC protocols.
You can add social features to your site by accessing the data shared by social networks. You can also tell these networks about a user's actions on your site so activities will show up on the social sites.
The OpenSocial REST and RPC protocols can be used to transmit user data, friend lists, and activities from a social site to your application. OAuth allows the user to authorize access to his or her data stored in social networks. The basic flow is:
There are several open source client libraries that make it easy to access the OpenSocial REST and RPC protocols in your favorite programming language.