Right now Facebook is the most popular website in the world. As part of there social web service they already declared a service called Facebook Connect. With this service we can authenticate user with there FB account and can get some information of particular user, as a result we can authenticate a user with his/her FB account and sign into our websites. Other famous services of FB is build custom Facebook Page, Aplication, Games etc.
[Facebook Connect SignIN]
Source code will posted soon.
SignIn with Facebook Implementation using Facebook Connect,