starting with version 3.1 (possibly 3.0, but i've only tested on 3.1.1) the iOS Facebook SDK disables the in-app UIWebView modal dialog (FBDialog) to authenticate the user and instead only enables iOS 6 system, Facebook.app and Mobile Safari. this hack was found and tested by stepping through the Facebook SDK source and building it manually into…

gistfile1.m

// GRITTY - force Facebook SDK 3.1+ to use in-app modal UIWebView for user authentication
@implementation FBSession (CLUAdditions)
- (void)HACKEDauthorizeWithPermissions:(NSArray*)permissions behavior:(FBSessionLoginBehavior)behavior defaultAudience:(FBSessionDefaultAudience)audience isReauthorize:(BOOL)isReauthorize {
    objc_msgSend(self, @selector(authorizeWithPermissions:defaultAudience:integratedAuth:FBAppAuth:safariAuth:fallback:isReauthorize:), permissions, audience, NO, NO, NO, YES, isReauthorize);
}
+ (void)load {
    method_exchangeImplementations(class_getInstanceMethod(self, @selector(authorizeWithPermissions:behavior:defaultAudience:isReauthorize:)), class_getInstanceMethod(self, @selector(HACKEDauthorizeWithPermissions:behavior:defaultAudience:isReauthorize:)));
}
@end

using the objective-c runtime to swap implementations instead of just a category leaves things less ambiguous.