Facebook is not only a social network, but also a software platform, and it allows to build applications on it. Many advanced features provided by Facebook to 3rd party sites (like Facebook Login, Facebook Like Button, etc) are implemented through Facebook Application.
If the above paragraph is a mumbo-jumbo for you, do not worry. The only thing you need to know about Facebook Applications is: you may need to get one for your site and it is very easy (no programming skills required).
After you set up custom domain for your daily-deal site, you will need to configure your Facebook Application in order to keep Facebook Login, Like button, Send button and other Facebook features working properly. It all can be done via "Facebook" section on the Settings page of the Admin section.
Here is a step-by-step guide how to set up Facebook application on your Getsocio-powered site.