OAuth is not the best choice for the given requirements because OAuth is an authorization framework rather than an authentication protocol. OAuth allows third-party applications to access a user's resources without sharing their credentials, focusing on providing access to resources and delegating permissions.
While OAuth is often used for integrating with social networks, it does not inherently authenticate users. OpenID, on the other hand, is designed specifically for user authentication and can seamlessly integrate with social networks. In this case, OpenID would be a more suitable federation method for the application, as it meets all the specified requirements, including reducing the number of credential requests, integrating with social networks, and authenticating users.
While SAML (Security Assertion Markup Language) is a widely used federation method for single sign-on (SSO) and can help reduce the number of credential requests, it is not the best choice for the given requirements, specifically for integrating with social networks.