Can't say as I've dreamt of online friends, but whenever I dream, there is always someone else with me that I can't identify.

People in our dreams don't have to resemble who we think we're dreaming about. I would pay more attention to the looks of the person in the dream. That is more telling of what your subconscious is dealing with. There must be something that the real online friend has touched in you; a memory, a feeling, something.

Listen to your dream and whatever feelings it evokes!