If you're not using a library which requires the -ObjC linker flag, you can remove -ObjC from your project: Build Settings > Other Linker Flags > remove -ObjC or if you are using a library which requires that flag, you can add the Facebook SDK.
In addition: There is a problem if you remove the -ObjC linker switch. The libCordova.a library specifies some categories on NSString which will not get loaded if -ObjC is removed. A work around is to remove libCordova.a from the linked frameworks and add this to the Other Linker Flags build setting:
-force_load $(BUILT_PRODUCTS_DIR)/libCordova.a