By becoming a single sign-on resource, Facebook also benefits because users become more reliant on the social network. Because you have the ability to use Facebook to sign in to all your favorite websites, you’re more likely to interact with Facebook more often. Even though you cannot access features of your Facebook account, and it wouldn’t necessarily make sense for you to have the ability to, it does increase the amount that you interact with Facebook, increasing the likelihood that you’ll actually head over to the social network and hang out over there as well.
Another beneficial feature of using Facebook Connect as a single sign on is that when you log in to Facebook either directly or connect with any site that uses Facebook Connect, Facebook signs you in to all the websites that you use Connect with. This means that if you visit five sites that use Facebook Connect, you will not be required to re-enter your login credentials multiple times. Also, that means that when you sign out of any one site, Facebook will sign you out of all websites. This can be helpful if you use a public computer so that you don’t have to remember to log in and out of all the websites that you may visit during a browsing session.
As Facebook continues to grow, the concept of single sign on using Facebook Connect will become one of the, if not the, most popular feature of Facebook Connect.
Source of Information : Facebook Marketing Designing Your Next Marketing Campaign