To integrate facebook in your iPhone application we recommend creating a wrapper class in c++ (and create an objective-c class inside the wrapper, handling the facebook API calls).
For example, you could create a facebookhelper class with 3 files:
facebookhelper.h (the class definition)
facebookhelper.mm (the implementation and objective-c API)
facebookhelper.cpp (an empty implementation)
You could add the .cpp file to the PC desktop project, and add the .mm file for the iPhone project. The header will be shared between the 2 implementations.