Facebook is an advertising platform, their main goal is to get new accounts and usage. They are going to get a lot of mileage with this via the API. Developers

Facebook is an advertising platform, their main goal is to get new accounts and usage. They are going to get a lot of mileage with this via the API. Developers will create useful and unexpected applications with this and in order to use the new apps you will need a facebook login. It generates new users for them and helps to prevent someone else from stealing their user base.