Facebook Connect is a single sign-on application which allows users to interact on other websites through their Facebook account. Launched in December 2008, Facebook Connect works with over 15,000 "partnered" sites to make site access easier for its users.
When a user chooses to access a third-party website through Facebook Connect, they allow that website to retrieve information they have given to Facebook, including their full name, pictures, wall posts, friend information, etc. This not only allows the user to skip the basic registration steps required by most websites, but it also allows the website to update the user's Facebook wall and news feed with their activities. By gaining access to the user's friends list, the website is able to show the user which of their friends have also accessed the website through Facebook Connect.
Facebook Help Centre answered frequently asked questions about Facebook Connect.
Facebook provides a guide on how to use Facebook Connect to personalize a website and expanding visitors.