Create a Facebook App and get the App ID
Some Facebook Widgets need and association with a Facebook App to work correctly. The association between widget and app allows the webmaster to use all the functionalities and opportunities the widgets itself offers.
Create an application
To get an App ID you have to creare a Facebook application. Fortunately the procedure is not so complicated.
Just go the Facebook Apps developer page and click on "My Apps" > "Add a New App", or press the green button on the left.
A overlay window will opens and ask you for some data about your new app. In the "Display Name" you have to insert a name for your app, so you can insert something like "My new app".
"Namespace" is optional, so we leave it empty.
We have to choose a "Category". If you will use your app just for the widgets on you web site you can choose almost any category, for example "Productivity".
Finally click on the "Create App ID" button.
Now your new app has been created and you can copy the App ID, a very long number, from your app list.