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There's nothing to stop a Facebook app being built like this. In fact, Facebook recommends this approach.

It's simply that many developers feel that it's easier to get all the permissions in one go, when the user first signs up.

Android, unfortunately does not have this functionality. All permissions have to be requested up front, whether or not they are needed for all users.

Cyanogen Mod, however, allows you to revoke permissions afterwards.

And most apps crash and burn when you do so, for obvious reasons.

Which is already too late.