FBgreeter is a WordPress plugin by axiom7 designed to open up your blog to more than 500 million Facebook users.
Facebook users will be able to log in to your blog using their Facebook account. This will allow them to comment and participate in other social features your blog may offer without having to go through the process of creating yet another user account.
The first time a Facebook user logs in to your blog using their Facebook account, FBgreeter will attempt to import their Facebook account in order to create a WordPress account automatically. This will allow you to keep track of your audience and it will allow your readers to log in to your blog without logging in to Facebook.
If FBgreeter creates a WordPress account on behalf of a Facebook user, FBgreeter will send a welcome message to that user with their login credentials.
All WordPress accounts created by FBgreeter will have the “Subscriber” role. Therefore, Facebook users will be able to manage their profile, read and write comments, receive newsletters, etc. No administrative, editorial or authoring rights will be granted ; you will keep full control of your blog.
Plugin homepage: http://axiom7.com/