Accessing Buddy List for Friends like Facebok

I'm trying to find out how Facebook gets access to a buddy list when a username and password is provided. I'm not sure if this is a WebAim thing or one of the other developer tools, sdks'.

Basically I'd want to use the same functionality as facebook, enter username and password, it returns a buddy list, then I check those names vs. the names in my system to find potential matches. Seems like an easy webservice would do the trick, but I'm sure there many authentication issues to worry about there.

Can someone point me in the right direction?

Thanks.