This blog will describe in details how to use Facebook as an identity provider to login into your SharePoint application.
There are many ways to integrate you SharePoint application with Facebook; so you can start developing authentication and authorization mechanisms for your SharePoint application, OR you can start using Windows Azure Access Control Service (ACS) that provides an easy way of authenticating users who need to access your SharePoint application.
How does ACS work with SharePoint and Facebook?
PS: I will assume you have ONLY a configured SharePoint machine and connected to the internet only.
First of all, let us create a windows azure account:
Go to Part 2
What are the steps if I want to make the identity provider to be another ASP.NET web application instead of facebook