Facebook is one of the most used social networking site today and is being explored by developers around the globe to create innovative & useful applications. Before starting to write the application, the first and foremost step is to register the application on facebook and get an Application ID and an API Key.
Facebook App ID and API key are two entirely different things. App ID is a unique number given to each application on facebook. API key however is used for making Facebook API calls.
The Facebook SDK refers to the API Key as the App Id and therefore makes one think that App ID is required to be passed where in actuality it expects the API Key. Passing the App Id will result in failing API calls.
To figure out how to find facebook App ID and API Key, check this.
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