facebook/facebook-ios-sdk — pull request #479 by joekr (2012-08-17 19:56:15+00:00)
Title: Fixed iOS 4.0 compatibility issue in FBDataDiskCache

'DISPATCH_QUEUE_PRIORITY_BACKGROUND' is not supported in iOS 4.0, only 5.0. It has been changed to 'DISPATCH_QUEUE_PRIORITY_LOW'