Facebook as one the biggest social media you would want to access as many APP as you want without any restrictions. The facebook API key is simply what you needed to use when you want to add a social button, widgets on your blog and this is inform of a code about 15 numbers.

For example, If you need to install Intensedebate comment system in your blog or other apps that will benefit your blog or website, then you need FB App ID or API Key, with this App, it will be easy for you to install any FB widget on your blog without looking for where to copy the code. This is no way hard to create, you just need to follow the steps am going to give you and in a minute you have your API key.

Here is what to do:

Go to [https://developers.facebook.com/apps] and click ‘Create New App’, then pick a name that suits your website for the Application Name.
Skip the namespace and web hosting options.(optional) Then Click “Continue” button.
Now go to Settings > Basic in the menu on the left and then click the “Website with Facebook Login” option on the right.
Then Enter your App Domains, Example: mywebsite.com
Now Enter your Site URL, Example: http://mywebsite.com/.
Then Hit the “Save Changes” button.
You can find App ID.
Just Copy your App ID and paste it were you need it.