According to Wikipedia:
Facebook Connect, also called Log in with Facebook, like OpenID, is a set of authentication APIs from Facebook that developers can use to help their users connect and share with such users’ Facebook friends (on and off Facebook) and increase engagement for their website or application. When so used, Facebook members can log on to third-party websites, applications, mobile devices and gaming systems with their Facebook identity and, while logged in, can connect with friends via these media and post information and updates to their Facebook profile. […] Log in with Facebook cannot be used by users in locations that cannot access Facebook, even if the third-party site is otherwise accessible from that location.
For those wishing to incorporate Facebook Connect or Log in with Facebook in their applications or website signup, visit Facebook’s developer guide.