I have implemented the facebook integration with the iOS version of cocos2dx. I’ll write a post in my blog about it soon (www.jesusbosch.com) and also here in the wiki. But basically you just need to install the native Facebook SDK for iOS, and put all the calls to the facebook api into the app delegate class. Maybe there is a cleaner way to do it, but with my limited knowledge of ObjectiveC is what I’ve been able to do. Then you just need to communicate to that class via Objective C++ wrappers.
On the Android side, I don’t know anything about it, I haven’t faced it yet, but will have to do it very soon.