NationBuilder comes integrated with a host of services out of the box, and we currently provide a Single Sign On (SSO) feature for people who want to log in to your nation using Facebook and Twitter. 

Anyone can click the "Sign in with Facebook" or "Sign in with Twitter" buttons to log in or create a profile on your sites. Where possible we will merge the information from that service into a user's existing account.

NationBuilder is not an SSO "provider." In other words, you cannot use your NationBuilder account to log into other services.