The first time you try to add Facebook photos you'll be presented with this authorisation dialog. To use Facebook as a photo source you need to authenticate John's Background Switcher so that it can access your Facebook profile. Note that John's Background Switcher won't share that information in any way. All you need to do is click the 'Authorise' button and you'll be presented with a Facebook logon page.
It'll ask you if you're happy for JBS to access the information it needs (really just you and your friends photos) and once you confirm it you're all done!
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