When I tested demo app in instruments leaks tool, I got GeneralBlock-XXXX that can be 56 or 2048. This happens after selecting the Publish button. (which actually calls [Facebook dialog:@"feed"]
It's consistent and reproduced on first attempt. (only on device, not on Simulator)
I also found this link reporting the same issue:
https://github.com/facebook/facebook-ios-sdk/issues/309
Thanks!