Yesterday I met with Kio Stark, an expert in stranger interaction design. We couldn't figure out what word you use for people who have become good friends on the internet. I asked twitter and got a mix of responses.
There were a few "efriends" suggestions, but overall the response was that internet friends can just be called friends. Ben Huh stated it really well when he said, "Friends with internets," which is to say they are friends.